Loop Fold Structure of Proteins: Resolution of Levinthal's Paradox

Abstract

According to Levinthal a protein chain of ordinary size would require enormous time to sort its conformational states before the final fold is reached. Experimentally observed time of folding suggests an estimate of the chain length for which the time would be sufficient. This estimate by order of magnitude fits to experimentally observed universal closed loop elements of globular proteins—25–30 residues.     

Segmental flexibility and head-head interaction in scallop myosin. A study using saturation transfer electron paramagnetic resonance spectroscopy

Saturation transfer electron paramagnetic resonance spectroscopy was used to investigate the rotational motion of the head domains of native and desensitized scallop myosin and its proteolytic subfragments. Scallop myosin was spin-labelled with 4-(2-iodoacetamido)-2,2,6,6-tetramethylpiperidinooxyl, which reacted with a heavy chain residue in the subfragment 1 domain. As previously shown for rabbit skeletal muscle myosin (Thomas et al., 1975), the two head domains of native scallop myosin appear to have independent motion (rotational correlation time, pi, = 0.8 X 10(-7) s for subfragment 1; 1.4 X 10(-7) s for myosin). However, removal of a regulatory light chain, to effect desensitization of the actin-activated ATPase, was associated with an increase in pi for myosin to a value of 2.4 X 10(-6) s. The Ca2+ sensitivity and initial correlation time were restored on recombination of the regulatory light chain in the presence of Mg2+. Sedimentation velocity profiles in an analytical ultracentrifuge indicated that the desensitized myosin preparations were largely monomeric and therefore the change in pi appears to reflect an intramolecular event. Addition of EDTA to spin-labelled scallop heavy meromyosin caused an immediate 2.5 to 4-fold increase in pi and a partial desensitization of the ATPase activity. Comparable experiments with subfragment 1 yielded a barely detectable increase in pi (1.5-fold) in the first ten minutes. The restricted rotational motion observed in desensitized myosin and heavy meromyosin could arise by a conformational change in the subfragment 1-subfragment 2 hinge region or by an association of one head with its partner. The latter mechanism, involving the exposed light chain binding site, would also explain the preferential release of one regulatory light chain from scallop myosin, and might account for some other co-operative effects observed in this molecule (Bagshaw, 1980).     

Electron diffraction analysis of the M412 intermediate of bacteriorhodopsin.

High resolution electron diffraction data have been recorded for glucose-embedded purple membrane specimens in which bacteriorhodopsin (bR) has been trapped by cooling slowly to below--100 degrees C under continuous illumination. Thin films (OD approximately 0.7) of glucose-embedded membranes, prepared as a control, showed virtually 100% conversion to the M state, and stacks of such thin film specimens gave very similar x-ray diffraction patterns in the bR568 and the M412 state in most experiments. To be certain that any measured differences in diffraction intensity would be real, two independent sets of electron diffraction intensities were recorded for near-equatorial, i.e. (hkO), reflections. Little correlation was indeed observed between these two sets for delta F values at low resolution (15-5.0 A, 49 reflections), but the correlation coefficient is approximately 0.3 at high resolution (5.0-3.3 A, 218 reflections). Thus, while most of the measured difference is error, the mean delta F and the correlation coefficient can be used to estimate the smaller, true delta F due to structural changes occurring in the M state. The magnitude of this estimated true mean delta F is equal to what would be produced if approximately five to seven nonhydrogen atoms were moved to structurally uncorrelated (i.e., new) positions in the M state. Movements of a few amino acid side chains, and repositioning of atoms of the retinal group and the associated lysine side chain after trans-cis isomerization, are the most probable causes of the observed intensity changes in the M state.(ABSTRACT TRUNCATED AT 250 WORDS)     

Chain scission of hyaluronan by peroxynitrite

The reaction of peroxynitrite with the biopolymer hyaluronan has been studied using stopped-flow techniques combined with detection of molecular weight changes using the combination of gel permeation chromatography and multiangle laser light scattering. From the effect of peroxynitrite on the yield of hyaluronan chain breaks, it was concluded that the chain breaks were caused by hydroxyl radicals which escape a cage containing the *OH NO*(2) radical pair. The yield of free hydroxyl radicals was determined as 5+/-1% (as a proportion of the total peroxynitrite concentration). At high peroxynitrite concentrations, it was observed that the yield of chain breaks leveled out, an effect largely attributable to the scavenging of hydroxyl radicals by nitrite ions present in the peroxynitrite preparation. These experiments also provided some support for a previous proposal that the adduct formed between ONOOH and ONOO(-) might itself produce hydroxyl radicals. The rate of this reaction would have to be of the order of 0.05 s(-1) to produce hydroxyl radical yields that would account quantitatively for chain break yields at high peroxynitrite concentrations. By carrying out experiments at higher hyaluronan concentrations, it was also concluded that an additional yield of chain breaks was produced by the bimolecular reaction of the polymer with ONOOH at a rate constant of about 10 dm(3)mol(-1)s(-1). At 5.3 x 10(-3)mol dm(-3) hyaluronan, this amounted to 3.5% chain breaks (per peroxynitrite concentration). These conclusions support the proposal that the yield of hydroxyl radicals arising from the isomerization of ONOOH to nitrate ions is relatively low.     

Estimating lead time and overdiagnosis associated with PSA screening from prostate cancer incidence trends

Summary .   The introduction of the prostate-specific antigen (PSA) test has led to dramatic changes in the incidence of prostate cancer in the United States. In this article, we use information on the increase and subsequent decline in prostate cancer incidence following the adoption of PSA to estimate the lead time associated with PSA screening. The lead time is a key determinant of the likelihood of overdiagnosis, one of the main costs associated with the PSA test. Our approach conceptualizes observed incidence as the sum of the secular trend in incidence, which reflects incidence in the absence of PSA, and the excess incidence over and above the secular trend, which is a function of population screening patterns and the unknown lead time. We develop a likelihood model for the excess incidence given the secular trend and use it to estimate the mean lead time under specified distributional assumptions. We also develop a likelihood model for observed incidence and use it to simultaneously estimate the mean lead time together with a smooth secular trend. Variances and confidence intervals are estimated via a parametric bootstrap. Our results indicate an average lead time of approximately 4.59 years (95% confidence interval [3.24, 5.93]) for whites and 6.78 years [5.42, 8.20] for blacks with a corresponding secular trend estimate that is fairly flat after the introduction of PSA screening. These estimates correspond to overdiagnosis frequencies of approximately 22.7% and 34.4% for screen-detected whites and blacks, respectively. Our results provide the first glimpse of a plausible secular trend in prostate cancer incidence and suggest that, in the absence of PSA screening, disease incidence would not have continued its historic increase, rather it would have leveled off in accordance with changes in prostate patterns of care unrelated to PSA.     

Maternal age-specific rates of 47,+21 and other cytogenetic abnormalities diagnosed in the first trimester of pregnancy in chorionic villus biopsy specimens: comparison with rates expected from observations at amniocentesis.

Results are presented on chromosome analyses made on 4,481 embryos or fetuses studied through chorionic villus sampling (CVS) in whom there was no known bias to presence of a chromosome abnormality except advanced parental age. We excluded from the analysis most cases in which mosaicism was diagnosed or in which there were cytogenetic discrepancies among samples obtained from the conceptus. There remain 48 cases of 47,+21, 39 cases of other nonlethal abnormalities, and 12 lethal abnormalities diagnosed in 4,481 studied. A regression analysis (restricted to the 3,848 cases diagnosed in the 35-49-year maternal age interval) was done on rates of (1) 47,+21, (2) other abnormalities excluding lethals or (3) including them, and (4) all abnormalities excluding lethals or (5) including them. The model used was y = exp(bx + c), where y is the rate of abnormality, x is maternal age at time of CVS (the modal age of the procedure was 10 gestational weeks from the last menstrual period), and b and c were, respectively, (1) 0.288 and -15.527; (2) 0.272 and -15.173; (3) 0.253 and -14.141; (4) 0.282 and -14.753; and (5) 0.271 and -14.195. We also derived rates of abnormalities at the time of CVS that would be predicted from rates (of nonmosaics) at amniocentesis after adjustment for the difference in gestational age between the usual times that these two procedures are done. The difference between the numbers of abnormalities predicted on the basis of these adjusted amniocentesis rates and the numbers observed at CVS provides an estimate of the spontaneous loss of embryos and fetuses between the usual gestational ages of these procedures. In these data, for 47,+21 the estimated proportion lost is 21% but the result is not significant at the .05 level. For other abnormalities excluding lethals the estimated spontaneous loss is 29% (P approximately .05); including lethals it is 44%. For all abnormalities, excluding lethals, pooled together, the estimate is 24%; including lethals it is 33%. The last three values are all significant at the .05 level or lower. The observed rates of abnormalities at CVS would be approximately 10% to 15% higher if one pooled diagnosed mosaics with the nonmosaics, but the estimated proportion of spontaneous fetal loss would be lower.     

Modelling studies on bovine spongiform encephalopathy occurrence to assist in the review of the over 30 months rule in Great Britain

The objective of this study was to contribute to a risk assessment to review the over 30 months (OTM) scheme in cattle, whereby all cattle aged over 30 months slaughtered in the UK are removed from the human food chain. We use back-calculation methods to estimate the impact of changes to the OTM rule, by using passive and active surveillance data collected between 1 July 2001 and 30 June 2002. There are two types of change considered: increasing the age limit and allowing animals born after a certain date into the food chain. Results indicate that under the OTM rule less than 1 animal in the last 12 months of the incubation period would enter the food chain in 2003. The birth date changes considered and small changes to the upper age limit would increase this number by a relatively small amount.     

Prevalence and phenotype consequence of FRAXA and FRAXE alleles in a large,ethnically diverse,special education-needs population.

We conducted a large population-based survey of fragile X (FRAXA) syndrome in ethnically diverse metropolitan Atlanta. The eligible study population consisted of public school children, aged 7-10 years, in special education-needs (SEN) classes. The purpose of the study was to estimate the prevalence among whites and, for the first time, African Americans, among a non-clinically referred population. At present, 5 males with FRAXA syndrome (4 whites and 1 African American), among 1,979 tested males, and no females, among 872 tested females, were identified. All males with FRAXA syndrome were mentally retarded and had been diagnosed previously. The prevalence for FRAXA syndrome was estimated to be 1/3,460 (confidence interval [CI] 1/7,143-1/1,742) for the general white male population and 1/4, 048 (CI 1/16,260-1/1,244) for the general African American male population. We also compared the frequency of intermediate and premutation FRAXA alleles (41-199 repeats) and fragile XE syndrome alleles (31-199 repeats) in the SEN population with that in a control population, to determine if there was a possible phenotype consequence of such high-repeat alleles, as has been reported previously. No difference was observed between our case and control populations, and no difference was observed between populations when the probands were grouped by a rough estimate of IQ based on class placement. These results suggest that there is no phenotype consequence of larger alleles that would cause carriers to be placed in an SEN class.     

The heavy chain of conventional kinesin interacts with the SNARE proteins SNAP25 and SNAP23

Recent studies on the conventional motor protein kinesin have identified a putative cargo-binding domain (residues 827-906) within the heavy chain. To identify possible cargo proteins which bind to this kinesin domain, we employed a yeast two-hybrid assay. A human brain cDNA library was screened, using as bait residues 814-963 of human ubiquitous kinesin heavy chain. This screen initially identified synaptosome-associated protein of 25 kDa (SNAP25) as a kinesin-binding protein. Subsequently, synaptosome-associated protein of 23 kDa (SNAP23), the nonneuronal homologue of SNAP25, was also confirmed to interact with kinesin. The sites of interaction, determined from in vivo and in vitro assays, are the N-terminus of SNAP25 (residues 1-84) and the cargo-binding domain of kinesin heavy chain (residues 814-907). Both regions are composed almost entirely of heptad repeats, suggesting the interaction between heavy chain and SNAP25 is that of a coiled-coil. The observation that SNAP23 also binds to residues 814-907 of heavy chain would indicate that the minimal kinesin-binding domain of SNAP23 and SNAP25 is most likely residues 45-84 (SNAP25 numbering), a heptad-repeat region in both proteins. The major binding site for kinesin light chain in kinesin heavy chain was mapped to residues 789-813 at the C-terminal end of the heavy chain stalk domain. Weak binding of light chain was also detected at the N-terminus of the heavy chain tail domain (residues 814-854). In support of separate binding sites on heavy chain for light chain and SNAPs, a complex of heavy and light chains was observed to interact with SNAP25 and SNAP23.     

Comparison of two methods of screening for genital chlamydial infection in women attending in general practice: cross sectional survey.

OBJECTIVES: To estimate the prevalence of Chlamydia trachomatis in asymptomatic women attending general practice: to assess the potential of the ligase chain reaction as a screening tool; and to evaluate selective screening criteria. DESIGN: Cross sectional survey. SETTING: Four general practices in northeast London. SUBJECTS: 890 women aged 18-35 years attending general practice for a cervical smear or a "young well woman" check between October 1994 and January 1996. The women were tested for C trachomatis with confirmed enzyme immunoassay (endocervical specimens) and ligase chain reaction assay on urine specimens. MAIN OUTCOME MEASURES: Prevalence of C trachomatis infection in women aged 18-35 on the basis of each test; sensitivity and specificity of both tests in this population. RESULTS: Prevalence of confirmed infection was 2.6% (95% confidence interval 1.6% to 3.6%) in all women. Prevalence on the basis of enzyme immunoassay was 1.6% (0.8% to 2.7%), with a sensitivity of 60% and a specificity of 100%. Prevalence on the basis of ligase chain reaction was 2.5% (1.5% to 3.9%), with 90% sensitivity and 99.8% specificity. Screening all women aged < or = 25 and all women who had had two or more partners in the past year would have detected 87% (20/23) of infections. CONCLUSION: Ligase chain reaction on urine samples performs at least as well as enzyme immunoassay on cervical specimens in this low prevalence population. It offers potential as a non-invasive screening tool. A simple selective screening strategy might be appropriate and would be able to detect most cases of infection. However, a rigorous economic evaluation of possible screening strategies is needed first.     

High-throughput evaluation of relative cell permeability between peptoids and peptides

Peptides are limited in their use as drugs due to low cell permeability and vulnerability to proteases. In contrast, peptoids are immune to enzymatic degradation and some peptoids have been shown to be relatively cell permeable. In order to facilitate future design of peptoid libraries for screening experiments, it would be useful to have a high-throughput method to estimate the cell permeability of peptoids containing different residues. In this paper, we report the strengths and limitations of a high-throughput cell-based permeability assay that registers the relative ability of steroid-conjugated peptides and peptoids to enter a cell. A comparative investigation of the physicochemical properties and side chain composition of peptoids and peptides is described to explain the observed higher cell permeability of peptoids over peptides. These data suggest that the conversion of the monomeric residues in peptides to an N-alkylglycine moiety in peptoids reduced the hydrogen-bonding potential of the molecules and is the main contributor to the observed permeability improvement.     

In VivoInterference of Paromomycin with Mitochondrial Activity ofLeishmania

Paromomycin is an aminocyclitol aminoglycoside antibiotic used for the treatment of leishmaniasis. In view of the central role of mitochondria in cellular energetics and metabolism, its effect onin vivomitochondrial activities ofLeishmania donovanipromastigotes—the parasite flagellate form—was investigated. The approach used flow cytometry, amperometric measure of O₂consumption, and, as a global estimate of mitochondrial dehydrogenases, thiazolyl blue reduction (MTT test); somein vitrocontrols were also made. When added to promastigote cultures for 24–72 h at 150–200 μM(= LC₅₀), paromomycin doubled the generation time, inhibited respiration, and lowered its associated electric potential difference across mitochondrial membranes, as measured by rhodamine 123 fluorescence. The chemical analogue neomycin was ineffective. Furthermore, thein vivomitochondrial dehydrogenase activities were lower, seemingly because of the shortage of respiratory substrates. Indeed, succinate addition to paromomycin-treated cultures partly restored mitochondrial membrane potential. However, no immediate effect of paromomycin on respiration was observed, neither inhibition of redox chain nor increase of membrane permeability (uncoupling). It is proposed that paromomycin acts at a metabolic level upstream of the respiratory chain itself. This would have the observed delayed consequence because the cell energy supply would progressively decline since it depends upon the proton gradient—viz., membrane potential—generated by respiration. In conclusion, paromomycin is an antibiotic affecting the cell's energetic metabolism; the respiratory dysfunction it induces may be a crucial aspect of its action againstLeishmaniaand possibly other cells.     

On the chain flexibility of arabinoxylans and other beta-(1-->4) polysaccharides

The recent paper by Dervilly-Pinel and co-workers (Carbohydr. Res. 2001, 330, 365-372) presents a complete macromolecular characterisation of a series of de-esterified arabinoxylans extracted and fractionated from wheat flour. From their measurements, they were able to extract parameters related to chain flexibility, such as the Mark-Houwink exponent a and the chain persistence length. However, the estimate they obtain for the latter parameter is rather larger than would be expected, since the arabinoxylan backbone is beta-(1-->4) like cellulose and the galactomannans. By treating their data in an alternative, but well accepted manner, we are able to obtain a lower value of persistence length, which agrees well with estimates for this parameter for cellulose, in the literature, and our own recent measurements for a series of differently substituted galactomannans. These results suggest that the parameters obtained may be applicable to other beta-(1-->4) polysaccharides.     

A neurocomputational model for optimal temporal processing

Humans can estimate the duration of intervals of time, and psychophysical experiments show that these estimations are subject to timing errors. According to standard theories of timing, these errors increase linearly with the interval to be estimated (Weber's law), and both at longer and shorter intervals, deviations from linearity are reported. This is not easily reconciled with the accumulation of neuronal noise, which would only lead to an increase with the square root of the interval. Here, we offer a neuronal model which explains the form of the error function as a result of a constrained optimization process. The model consists of a number of synfire chains with different transmission times, which project onto a set of readout neurons. We show that an increase in the transmission time corresponds to a superlinear increase of the timing errors. Under the assumption of a fixed chain length, the experimentally observed error function emerges from optimal selection of chains for each given interval. Furthermore, we show how this optimal selection could be implemented by competitive spike-timing dependent plasticity in the connections from the chains to the readout network, and discuss implications of our model on selective temporal learning and possible neural architectures of interval timing.     

Conformational equilibration time of unfolded protein chains and the folding speed limit

The speed with which the conformers of unfolded protein chains interconvert is a fundamental question in the study of protein folding. Kinetic evidence is presented here for the time constant for interconversion of disparate unfolded chain conformations of a small globular protein, cytochrome c, in the presence of guanidine hydrochloride denaturant. The axial binding reactions of histidine and methionine residues with the Fe(II) heme cofactor were monitored with time-resolved magnetic circular dichroism spectroscopy after photodissociation of the CO complexes of unfolded protein obtained from horse and tuna and from several histidine mutants of the horse protein. A kinetic model fitting both the reaction rate constants and spectra of the intermediates was used to obtain a quantitative estimate of the conformational diffusion time. The latter parameter was approximated as a first-order time constant for exchange between conformational subensembles presenting either a methionine or a histidine residue to the heme iron for facile binding. The mean diffusional time constant of the wild type and variants was 3 +/- 2 mus, close to the folding "speed limit". The implications of the relatively rapid conformational equilibration time observed are discussed in terms of the energy landscape and classical pathway time regimes of folding, for which the conformational diffusion time can be considered a pivot point.     

Discrete-time nonparametric estimation for semi-Markov models of chain-of-events data subject to interval censoring and truncation

Chain-of-events data are longitudinal observations on a succession of events that can only occur in a prescribed order. One goal in an analysis of this type of data is to determine the distribution of times between the successive events. This is difficult when individuals are observed periodically rather than continuously because the event times are then interval censored. Chain-of-events data may also be subject to truncation when individuals can only be observed if a certain event in the chain (e.g., the final event) has occurred. We provide a nonparametric approach to estimate the distributions of times between successive events in discrete time for data such as these under the semi-Markov assumption that the times between events are independent. This method uses a self-consistency algorithm that extends Turnbull's algorithm (1976, Journal of the Royal Statistical Society, Series B 38, 290-295). The quantities required to carry out the algorithm can be calculated recursively for improved computational efficiency. Two examples using data from studies involving HIV disease are used to illustrate our methods.     

Past states of continuous-time Markov models for ecological communities

Discrete-time Markov chains are often used to model communities of sessile organisms. The community is described by a set of discrete states, which may represent species or groups of species. Transitions between states are modelled using a stochastic matrix. A recent study showed how the time-reversal of such a Markov chain can be used to estimate the distribution of time since the last occurrence of some state of interest (such as empty space) at a point, given the current state of the point. However, if the underlying process operates in continuous time but is observed at regular intervals, this distribution describes the time since the last possible observation of the state of interest, rather than the time since its last occurrence. We show how to obtain the distribution of time since the last occurrence of a state of interest for a continuous-time homogeneous Markov chain. The expected time since the last occurrence of an initial state can be interpreted as a measure of the successional rank of a state. We show how to distinguish between different ways in which a state can have high successional rank. We apply our results to a marine subtidal community.     

Wild turkey (<Emphasis Type="Italic">Meleagris gallopavo</Emphasis>) detectability from helicopters and ramifications for estimating abundance

Aerial surveys have been used to estimate abundance for several wild bird species but its application for wild turkey (Meleagris gallopavo) populations has been limited. We surveyed Rio Grande wild turkey (M. gallopavo intermedia) populations during March 2006 using an R44 helicopter. We used flocks with radio-tagged birds to estimate flock detectability. We also used simulations to evaluate accuracy and precision and examine power to detect trends in population change. We observed that wild turkey flock detectability was 94.7% (74.0–99.9%; 95% CI). Our simulations suggested helicopter surveys would underestimate abundance by about 5.6% (4.6% CV). Surveying 980 to 1,960 km² (requiring 27 to 55 h of flight time) can provide sufficient power (≥0.80) to detect a 10 to 25% change in abundance over a 4- to 5-year period.     

Nucleotide sequence of complementary DNA and derived amino acid sequence of murine complement protein C3

The nucleotide sequences coding for murine complement component C3 have been determined from a cloned genomic DNA fragment and several overlapping cloned complementary DNA fragments. The amino acid sequence of the protein was deduced. The mature beta and alpha subunits contain 642 and 993 amino acids respectively. Including a 24 amino acid signal peptide and four arginines in the beta-alpha transition region, which are probably not contained in the mature protein, the unglycosylated single chain precursor protein preproC3 would have a molecular mass of 186 484 Da and consist of 1663 amino acid residues. The C3 messenger RNA would be composed of a 56 +/- 2 nucleotide long 5' non-translated region, 4992 nucleotides of coding sequence, and a 3' non-translated region of 39 nucleotides, excluding the poly A tail. The beta chain contains only three cysteine residues, the alpha chain 24, ten of which are clustered in the carboxy terminal stretch of 175 amino acids. Two potential carbohydrate attachment sites are predicted for the alpha chain, none for the beta chain. From a comparison with human C3 cDNA sequence (of which over 80% has been determined) an extensive overall sequence homology was observed. Human and murine preproC3 would be of very similar length and share several noteworthy properties: the same order of the subunits in the precursor, the same basic residue multiplet in the beta-alpha transition region, and a glutamine residue in the thioester region. The equivalent position of the known factor I cleavage sites in human C3 alpha could be located in the murine C3 alpha chain and the size and sequence of the resulting peptide were deduced. A comparison of the amino acid sequences of murine C3 and human alpha 2-macroglobulin is given. Several areas of strong sequence homology are observed, and we conclude that the two genes must have evolved from a common ancestor.     

Close genetic relationship between cultivated and natural populations of common buckwheat in the Sanjiang area is not due to recent gene flow between them-An analysis using microsatellite markers

Natural populations of wild common buckwheat have been found growing adjacent to cultivated populations of common buckwheat. Gene flow between the cultivated and natural populations would be expected in such cases. To evaluate the amount of gene flow, two sets composed of a cultivated buckwheat population and an adjacent natural population of wild common buckwheat were chosen, one from Yanjing village in the Sanjiang area, which is presumed to be the original birthplace of common buckwheat, and one from Jinhe village, Yanyuan district of Sichuan province in China. The genotypes of 45 individuals from each population were examined at eight microsatellite marker loci to estimate the magnitude of gene flow between the cultivated and wild common buckwheat populations. The Bayesian method with a Markov chain Monte Carlo approach estimated that the magnitude of gene flow between the populations in the Sanjiang area at 0.002-0.008 was not significantly different from that found in Yanyuan district at 0.002-0.008. The gene flow between cultivated populations was higher, usually at 0.002-0.044 (exceptionally high at 0.255 between cultivated populations of Yanjing and Jinhe), than that found between a cultivated population and a natural population (0.002-0.008) or between two natural populations (0.002-0.003). Therefore, the genetic similarity found between the cultivated populations and natural populations observed in the Sanjiang area (Konishi et al., 2005) was not due to recent gene flow between them. This in turn suggests that the close genetic relationship found in the Sanjiang area may be due to the common ancestry of the natural populations and cultivated common buckwheat.