共查询到20条相似文献,搜索用时 125 毫秒
1.
To characterize the coding-sequence divergence of closely related genomes, we compared DNA sequence divergence between sequences
from a Brassica rapa ssp. pekinensis EST library isolated from flower buds and genomic sequences from Arabidopsis thaliana. The specific objectives were (i) to determine the distribution of and relationship between K
a and K
s, (ii) to identify genes with the lowest and highest K
a:K
s values, and (iii) to evaluate how codon usage has diverged between two closely related species. We found that the distribution
of K
a:K
s was unimodal, and that substitution rates were more variable at nonsynonymous than synonymous sites, and detected no evidence
that K
a and K
s were positively correlated. Several genes had K
a:K
s values equal to or near zero, as expected for genes that have evolved under strong selective constraint. In contrast, there
were no genes with K
a:K
s >1 and thus we found no strong evidence that any of the 218 sequences we analyzed have evolved in response to positive selection.
We detected a stronger codon bias but a lower frequency of GC at synonymous sites in A. thaliana than B. rapa. Moreover, there has been a shift in the profile of most commonly used synonymous codons since these two species diverged
from one another. This shift in codon usage may have been caused by stronger selection acting on codon usage or by a shift
in the direction of mutational bias in the B. rapa phylogenetic lineage. 相似文献
2.
Employing a set of 43 othologous mouse and rat genes, Hughes and Yeager (J. Mol. Evol. 45:125–130, 1997) reported (1) no correlation between synonymous and nonsynonymous rates of nucleotide substitution, (2)
a positive correlation between intronic GC contents (GC
i) and intronic substitution rates (K
i), (3) that the average K
i value was very similar to the average K
s value, and (4) that the compositional correlation between the rat and the mouse genes is stronger at the third codon position
(GC3) than at the first and second codon positions (GC12). We have examined the robustness of these results to alterations in substitution rate estimation protocol, alignment protocol,
and statistical procedure. We find that a significant correlation between K
a and K
s is observed either if a rank correlation statistic is used instead of regression analysis, if one outlier is excluded from
the analysis, or if a regression weighted by gene size is employed. The correlation between K
i and GC
i we find to be sensitive to changes in alignment protocol and disappears on the use of weighted means. The finding that K
s and K
i are approximately the same is dependent on the method for estimating K
s values. Finally, the variance around the regression line of rat GC3 versus mouse GC3 we find to be significantly higher than that in GC12. The source of the discrepancy between this and Hughes and Yeager's result is unclear. The variance around the line for GC4 is higher still, as might be expected. Using a methodology that may be considered preferable to that of Hughes and Yeager,
we find that all four of their results are contradicted. More importantly this analysis reinforces the need for caution in
assembling and analyzing data sets, as the degree of sensitivity to what many might consider minor methodological alterations
is unexpected.
Received: 2 February 1998 / Accepted: 23 March 1998 相似文献
3.
The idea that the pattern of point mutation in Drosophila has remained constant during the evolution of the genus has recently been challenged. A study of the nucleotide composition
focused on the Drosophila
saltans group has evidenced unsuspected nucleotide composition differences among lineages. Compositional differences are associated
with an accelerated rate of amino acid replacement in functionally less constrained regions. Here we reassess this issue from
a different perspective. Adopting a maximum-likelihood estimation approach, we focus on the different predictions that mutation
and selection make about the nonsynonymous-to-synonymous rate ratio. We investigate two gene regions, alcohol dehydrogenase
(Adh) and xanthine dehydrogenase (Xdh), using a balanced data set that comprises representatives from the melangaster, obscura, saltans, and willistoni groups. We also consider representatives of the Hawaiian picture-winged group. These Hawaiian species are known to have experienced
repeated bottlenecks and are included as a reference for comparison. Our results confirm patterns previously detected. The
branch ancestral to the fast-evolving willistoni/saltans lineage, where most of the change in GC content has occurred, exhibits an excess of synonymous substitutions. The shift in
mutation bias has affected the extent of the rate variation among sites in Xdh.
Received: 4 May 1999 / Accepted: 26 July 1999 相似文献
4.
Murid rodents show much less variation in isochore base composition than do most other mammals, a difference which has been
referred to as the murid shift. We have investigated the murid shift by asking (1) whether the murid shift is ongoing and
(2) whether there is any evidence of selection or biased gene conversion affecting base composition in the present-day mouse
genome. By estimating the ancestral base composition of protein-coding genes in murids we can confirm that the murid shift
is ongoing. Tests using nongenic polymorphism data fail to reject the hypothesis that base composition is due to mutation
bias alone. However, the patterns of compositional change suggested by the polymorphism and divergence data differ, suggesting
the possibility of two murid shifts. 相似文献
5.
Synonymous codon usage in related species may differ as a result of variation in mutation biases, differences in the overall
strength and efficiency of selection, and shifts in codon preference—the selective hierarchy of codons within and between
amino acids. We have developed a maximum-likelihood method to employ explicit population genetic models to analyze the evolution
of parameters determining codon usage. The method is applied to twofold degenerate amino acids in 50 orthologous genes from
D. melanogaster and D. virilis. We find that D. virilis has significantly reduced selection on codon usage for all amino acids, but the data are incompatible with a simple model
in which there is a single difference in the long-term N
e, or overall strength of selection, between the two species, indicating shifts in codon preference. The strength of selection
acting on codon usage in D. melanogaster is estimated to be |N
e
s|≈ 0.4 for most CT-ending twofold degenerate amino acids, but 1.7 times greater for cysteine and 1.4 times greater for AG-ending
codons. In D. virilis, the strength of selection acting on codon usage for most amino acids is only half that acting in D. melanogaster but is considerably greater than half for cysteine, perhaps indicating the dual selection pressures of translational efficiency
and accuracy. Selection coefficients in orthologues are highly correlated (ρ= 0.46), but a number of genes deviate significantly
from this relationship.
Received: 20 December 1998 / Accepted: 17 February 1999 相似文献
6.
Héctor Musto Héctor Romero Helena Rodríguez-Maseda 《Journal of molecular evolution》1998,46(2):159-167
Synonymous codon choices vary considerably among Schistosoma mansoni genes. Principal components analysis detects a single major trend among genes, which highly correlates with GC content in
third codon positions and exons, but does not discriminate among putatively highly and lowly expressed genes. The effective
number of codons used in each gene, and its distribution when plotted against GC3, suggests that codon usage is shaped mainly by mutational biases. The GC content of exons, GC3, 5′, 3′, and flanking (5′+ 3′+ introns) regions are all correlated among them, suggesting that variations in GC content may
exist among different regions of the S. mansoni genome. We propose that this genome structure might be among the most important factors shaping codon usage in this species,
although the action of selection on certain sequences cannot be excluded.
Received: 10 March 1997 / Accepted: 27 June 1997 相似文献
7.
Charles M. Matthews Cassandra J. Vandenberg Clive N.A. Trotman 《Journal of molecular evolution》1998,46(6):729-733
The Artemia hemoglobin is a dimer comprising two nine-domain covalent polymers in quaternary association. Each polymer is encoded by
a gene representing nine successive globin domains which have different sequences and are presumed to have been copied originally
from a single-domain gene. Two different polymers exist as the result of a complete duplication of the nine-domain gene, allowing
the formation of either homodimers or the heterodimer. The total population size of 18 domains comprising nine corresponding
pairs, coupled with the probability that they reflect several hundred million years of evolution in the same lineage, provides
a unique model in which the process of gene multiplication can be analyzed. The outcome has important implications for the
reliability of local molecular clocks.
The two polymers differ from each other at 11.7% of amino acid sites; however when corresponding individual domains are compared
between polymers, amino acid substitution fluctuates by a factor of 2.7-fold from lowest to highest. This variation is not
obvious at the DNA level: Domain pair identity values fluctuate by 1.3-fold. Identity values are, however, uncorrected for
multiple substitutions, and both silent and nonsilent changes are pooled. Therefore, to determine the variability in relative
substitution rates at the DNA level, we have used the method of Li (1993, J Mol Evol 36:96–99) to determine estimates of nonsynonymous (K
A
) and synonymous (K
S
) substitutions per site for the nine pairs of domains. As expected, the overall level of silent substitutions (K
S
of 56.9%) far exceeded nonsilent substitutions (K
A
of 6.7%); however, for corresponding domain pairs, K
A
fluctuates by 2.3-fold and K
S
by 1.7-fold. The large discrepancies reflected in the expressed protein have accrued within a single lineage and the implication
is that divergence dates of different genera based on amino acid sequences, even with well-studied proteins of reasonable
size, can be wrong by a factor well in excess of 2.
Received: 4 June 1997 / Accepted: 17 December 1997 相似文献
8.
Fernando Alvarez-Valin Kamel Jabbari Nicolas Carels Giorgio Bernardi 《Journal of molecular evolution》1999,49(3):330-342
In this work, we have investigated the relationships between synonymous and nonsynonymous rates and base composition in coding
sequences from Gramineae to analyze the factors underlying the variation in substitutional rates. We have shown that in these genes the rates of nucleotide
divergence, both synonymous and nonsynonymous, are, to some extent, dependent on each other and on the base composition. In
the first place, the variation in nonsynonymous rate is related to the GC level at the second codon position (the higher the
GC2 level, the higher the amino acid replacement rate). The correlation is especially strong with T2, the coefficients being significant in the three data sets analyzed. This correlation between nonsynonymous rate and base
composition at the second codon position is also detectable at the intragenic level, which implies that the factors that tend
to increase the intergenic variance in nonsynonymous rates also affect the intragenic variance. On the other hand, we have
shown that the synonymous rate is strongly correlated with the GC3 level. This correlation is observed both across genes and at the intragenic level. Similarly, the nonsynonymous rate is also
affected at the intragenic level by GC3 level, like the silent rate. In fact, synonymous and nonsynonymous rates exhibit a parallel behavior in relation to GC3 level, indicating that the intragenic patterns of both silent and amino acid divergence rates are influenced in a similar
way by the intragenic variation of GC3. This result, taken together with the fact that the number of genes displaying intragenic correlation coefficients between
synonymous and nonsynonymous rates is not very high, but higher than random expectation (in the three data sets analyzed),
strongly suggests that the processes of silent and amino acid replacement divergence are, at least in part, driven by common
evolutionary forces in genes from Gramineae.
Received: 2 July 1998 / Accepted: 18 April 1999 相似文献
9.
Detailed nucleotide diversity studies revealed that the fil1 gene of Antirrhinum, which has been reported to be single copy, is a member of a gene family composed of at least five genes. In four Antirrhinum majus populations with different mating systems and one A. graniticum population, diversity within populations is very low. Divergence among Antirrhinum species and between Antirrhinum and Digitalis is also low. For three of these genes we also obtained sequences from a more divergent member of the Scrophulariaceae, Verbascum nigrum. Compared with Antirrhinum, little divergence is again observed. These results, together with similar data obtained previously for five cycloidea genes,
suggest either that these gene families (or the Antirrhinum genome) are unusually constrained or that there is a low rate of substitution in these lineages. Using a sample of 52 genes,
based on two measures of codon usage (ENC and GC3 content), we show that cyc and fil1 are among the least biased Antirrhinum genes, so that their low diversity is not due to extreme codon bias.
Received: 20 June 2000 / Accepted: 25 October 2000 相似文献
10.
Lars S. Jermiin Peter G. Foster Dan Graur Roger M. Lowe Ross H. Crozier 《Journal of molecular evolution》1996,42(4):476-480
The most generally applicable procedure for obtaining estimates of the symmetrical, or strand-nonspecific, directional mutation
pressure (μD) on protein-coding DNA sequences is to determine the G+C content at synonymous codon sites (P
syn), and to divide P
syn by twice the arithmetic mean of the G+C content at synonymous codon sites of a large number of randomly generated, synonymously
coding DNA sequences (P
syn). Unfortunately, the original procedure yields biased estimates of P
syn and μD and is computationally expensive. We here present a fast procedure for estimating unbiased μD values. The procedure employs direct calculation of P
syn (≈P
syn) and two normalization procedures, one for P
syn≤P
syn and another for P
syn≥P
syn. The normalization removes a bias sometimes caused by codons specifying arginine, asparagine, isoleucine, and leucine. Consequently,
comparison of protein-coding genes that are translated using different genetic codes is facilitated.
Received: 5 May 1995 / Accepted: 30 November 1995 相似文献
11.
We investigated the block of KATP channels by glibenclamide in inside-out membrane patches of rat flexor digitorum brevis muscle.
(1) We found that glibenclamide inhibited KATP channels with an apparent K
i
of 63 nm and a Hill coefficient of 0.85. The inhibition of KATP channels by glibenclamide was unaffected by internal Mg2+.
(2) Glibenclamide altered all kinetic parameters measured; mean open time and burst length were reduced, whereas mean closed
time was increased.
(3) By making the assumption that binding of glibenclamide to the sulphonylurea receptor (SUR) leads to channel closure, we
have used the relation between mean open time, glibenclamide concentration and K
D
to estimate binding and unbinding rate constants. We found an apparent rate constant for glibenclamide binding of 9.9 × 107
m
−1 sec−1 and an unbinding rate of 6.26 sec−1.
(4) Glibenclamide is a lipophilic molecule and is likely to act on sulfonylurea receptors from within the hydrophobic phase
of the cell membrane. The glibenclamide concentration within this phase will be greater than that in the aqueous solution
and we have taken this into account to estimate a true binding rate constant of 1.66 × 106
m
−1 sec−1.
Received: 7 July 1996/Revised: 4 October 1996 相似文献
12.
The current through TOK1 (YKC1), the outward-rectifying K+ channel in Saccharomyces cerevisiae, was amplified by expressing TOK1 from a plasmid driven by a strong constitutive promoter. TOK1 so hyper-expressed could overcome the K+ auxotrophy of a mutant missing the two K+ transporters, TRK1 and TRK2. This trk1Δtrk2Δ double mutant hyperexpressing the TOK1 transgene had a higher internal K+ content than one expressing the empty plasmid. We examined protoplasts of these TOK1-hyperexpressing cells under a patch clamp. Besides the expected K+ outward current activating at membrane potential (V
m
) above the K+ equilibrium potential (E
K+
), a small inward current was consistently observed when the V
m
was slightly below E
K+
. The inward and the outward currents are similar in their activation rates, deactivation rates, ion specificities and Ba2+ inhibition, indicating that they flow through the same channel. Thus, the yeast outwardly rectifying K+ channel can take up K+ into yeast cells, at least under certain conditions.
Received: 1 October 1998/Revised: 9 December 1998 相似文献
13.
The general purpose of this theoretical work is to contribute to understand the physiological role of the electrogenic properties
of the sodium pump, by studying a dynamic model that integrates diverse processes of ionic and water transport across the
plasma membrane. For this purpose, we employ a mathematical model that describes the rate of change of the intracellular concentrations
of Na+, K+ and Cl−, of the cell volume, and of the plasma membrane potential (V
m
). We consider the case of a nonexcitable, nonpolarized cell expressing the sodium pump; Na+, K+, Cl− and water channels, and cotransporters of KCl and NaCl in its plasma membrane. We particularly analyze here the conditions
under which the physiological V
m
can be generated in a predominantly electrogenic fashion, as a result of the activity of the sodium pump. A major conclusion
of this study is that, for the cell model considered, a low potassium permeability is not a sufficient condition for a predominantly
electrogenic generation of the V
m
by the sodium pump. The presence of an electroneutral exchange of Na+ and K+ represents a necessary additional requirement.
Received: 8 September 1999/Revised: 21 March 2000 相似文献
14.
Hyperpolarization-activated K channels (K
H
channels) in the plasmalemma of guard cells operate at apoplastic pH range of 5 to over 7. Using patch clamp in a whole-cell
mode, we characterized the effect of varying the external pH between 4.4–8.1 on the activity of the K
H
channels in isolated guard cell protoplasts from Vicia faba leaves.
Acidification from pH 5.5 to 4.4 increased the macroscopic conductance of the K
H
channels by 30–150% while alkalinization from pH 5.5 to 8.1 decreased it only by roughly 15%. The voltage-independent maximum cell conductance, increased by ∼60% between pH 8.1 and 4.4 with an apparent pK
a
of 5.3, most likely owing to the increased availability of channels. Voltage-dependent gating was affected only between pH 5.5 and 4.4. Acidification in this range shifted the voltage-dependent open probability by over 10 mV. We interpret this shift as an increase of the electrical field sensed by the gating subunits
caused by the protonation of external negative surface charges. Within the framework of a surface charge model the mean spacing
of these charges was ∼30 ? and their apparent dissociation constant was 10−4.6. The overall voltage sensitivity of gating was not altered by pH changes. In a subgroup of protoplasts analyzed within the
framework of a Closed-Closed-Open model, the effect of protons on gating was limited to shifting of the voltage-dependence
of all four transition rate constants.
Received: 26 April 1996/Revised: 29 June 1996 相似文献
15.
Three types of electrical excitation have been investigated in the marine diatom Coscinodiscus wailesii. I: Depolarization-triggered, transient Cl− conductance, G
Cl
(t), followed by a transient, voltage-gated K+ conductance, G
K
, with an active state a and two inactive states i
1 and i
2 in series (a-i
1-i
2). II: Similar G
Cl
(t) as in Type-I but triggered by hyperpolarization; a subsequent increase of G
K
in this type is indicated but not analyzed in detail. III: Hyperpolarization-induced transient of a voltage-gated activity
of an electrogenic pump (i
2-a-i
2), followed by G
Cl
(t) as in Type-II excitations. Type-III with pump gating is novel as such. G
Cl
(t) in all types seems to reflect the mechanism of InsP−
3 and Ca2+-mediated G
Cl
(t) in the action potential in Chara (Biskup et al., 1999). The nonlinear current-voltage-time relationships of Type-I and Type-III excitations have been recorded
under voltage-clamp using single saw-tooth command voltages (voltage range: −200 to +50 mV, typical slope: ±1 Vs−1). Fits of the corresponding models to the experimental data provided numerical values of the model parameters. The statistical
significance of these solutions is investigated. We suggest that the original function of electrical excitability of biological
membranes is related to osmoregulation which has persisted through evolution in plants, whereas the familiar and osmotically
neutral action potentials in animals have evolved later towards the novel function of rapid transmission of information over
long distances.
Received: 2 December 1999/Revised: 3 March 2000 相似文献
16.
In many unicellular organisms, invertebrates, and plants, synonymous codon usage biases result from a coadaptation between
codon usage and tRNAs abundance to optimize the efficiency of protein synthesis. However, it remains unclear whether natural
selection acts at the level of the speed or the accuracy of mRNAs translation. Here we show that codon usage can improve the
fidelity of protein synthesis in multicellular species. As predicted by the model of selection for translational accuracy,
we find that the frequency of codons optimal for translation is significantly higher at codons encoding for conserved amino
acids than at codons encoding for nonconserved amino acids in 548 genes compared between Caenorhabditis elegans and Homo sapiens. Although this model predicts that codon bias correlates positively with gene length, a negative correlation between codon
bias and gene length has been observed in eukaryotes. This suggests that selection for fidelity of protein synthesis is not
the main factor responsible for codon biases. The relationship between codon bias and gene length remains unexplained. Exploring
the differences in gene expression process in eukaryotes and prokaryotes should provide new insights to understand this key
question of codon usage.
Received: 18 June 2000 / Accepted: 10 November 2000 相似文献
17.
D. Casane S. Boissinot B.H.-J. Chang L.C. Shimmin W.-H. Li 《Journal of molecular evolution》1997,45(3):216-226
We sequenced three argininosuccinate-synthetase-processed pseudogenes (ΨAS-A1, ΨAS-A3, ΨAS-3) and their noncoding flanking
sequences in human, orangutan, baboon, and colobus. Our data showed that these pseudogenes were incorporated into the genome
of the Old World monkeys after the divergence of the Old World and New World monkey lineages. These pseudogene flanking regions
show variable mutation rates and patterns. The variation in the G/C to A/T mutation rate (u) can account for the unequal GC contents at equilibrium: 34.9, 36.9, and 41.7% in the pseudogene ΨAS-A1, ΨAS-A3, and ΨAS-3
flanking regions, respectively. The A/T to G/C mutation rate (v) seems stable and the u/v ratios equal 1.9, 1.7, and 1.4 in the flanking regions of ΨAS-A1, ΨAS-A3, and ΨAS-3, respectively. These ``regional' variations
of the mutation rate affect the evolution of the pseudogenes, too. The ratio u/v being greater than 1.0 in each case, the overall mutation rate in the GC-rich pseudogenes is, as expected, higher than in
their GC-poor flanking regions. Moreover, a ``sequence effect' has been found. In the three cases examined u and v are higher (at least 20%) in the pseudogene than in its flanking region—i.e., the pseudogene appears as mutation ``hot'
spots embedded in ``cold' regions. This observation could be partly linked to the fact that the pseudogene flanking regions
are long-standing unconstrained DNA sequences, whereas the pseudogenes were relieved of selection on their coding functions
only around 30–40 million years ago. We suspect that relatively more mutable sites maintained unchanged during the evolution
of the argininosuccinate gene are able to change in the pseudogenes, such sites being eliminated or rare in the flanking regions
which have been void of strong selective constraints over a much longer period. Our results shed light on (1) the multiplicity
of factors that tune the spontaneous mutation rate and (2) the impact of the genomic position of a sequence on its evolution.
Received: 10 February 1997 / Accepted: 21 April 1997 相似文献
18.
Analysis of the mechanistic basis by which sodium-coupled transport systems respond to changes in membrane potential is inherently
complex. Algebraic expressions for the primary kinetic parameters (K
m
and V
max
) consist of multiple terms that encompass most rate constants in the transport cycle. Even for a relatively simple cotransport
system such as the Na+/alanine cotransporter in LLC-PK1 cells (1:1 Na+ to substrate coupling, and an ordered binding sequence), the algebraic expressions for K
m
for either substrate includes ten of the twelve rate constants necessary for modeling the full transport cycle. We show here
that the expression of K
m
of the first-bound substrate (Na+) simplifies markedly if the second-bound substrate (alanine) is held at a low concentration so that its' binding becomes
the rate limiting step. Under these conditions, the expression for the K
Na
m
includes rate constants for only two steps in the full cycle: (i) binding/dissociation of Na+, and (ii) conformational `translocation' of the substrate-free protein. The influence of imposed changes in membrane potential
on the apparent K
Na
m
for the LLC-PK1 alanine cotransporter at low alanine thus provides insight to potential dependence at these sites. The data show no potential
dependence for K
Na
m
at 5 μm alanine, despite marked potential dependence at 2 mm alanine when the full algebraic expression applies. The results suggest that neither translocation of the substrate-free
form of the transporter nor binding/dissociation of extracellular sodium are potential dependent events for this transport
system.
Received: 10 April 1998/Revised: 6 July 1998 相似文献
19.
In bacteria, synonymous codon usage can be considerably affected by base composition at neighboring sites. Such context-dependent
biases may be caused by either selection against specific nucleotide motifs or context-dependent mutation biases. Here we
consider the evolutionary conservation of context-dependent codon bias across 11 completely sequenced bacterial genomes. In
particular, we focus on two contextual biases previously identified in Escherichia coli; the avoidance of out-of-frame stop codons and AGG motifs. By identifying homologues of E. coli genes, we also investigate the effect of gene expression level in Haemophilus influenzae and Mycoplasma genitalium. We find that while context-dependent codon biases are widespread in bacteria, few are conserved across all species considered.
Avoidance of out-of-frame stop codons does not apply to all stop codons or amino acids in E. coli, does not hold for different species, does not increase with gene expression level, and is not relaxed in Mycoplasma spp., in which the canonical stop codon, TGA, is recognized as tryptophan. Avoidance of AGG motifs shows some evolutionary
conservation and increases with gene expression level in E. coli, suggestive of the action of selection, but the cause of the bias differs between species. These results demonstrate that
strong context-dependent forces, both selective and mutational, operate on synonymous codon usage but that these differ considerably
between genomes.
Received: 6 May 1999 / Accepted: 29 October 1999 相似文献
20.
The anion conductance of the plasma membrane of Coffea arabica protoplasts was isolated and characterized using the whole-cell patch clamp technique. Voltage pulse protocols revealed two
components: a voltage-gated conductance (G
s
) and a voltage-independent one (G
l
). G
s
is activated upon depolarization (e-fold activation every +36 mV) with time constants of 1 sec and 5 sec at all potentials.
G
l
and G
s
also differ by their kinetic and biophysical properties. In bi-ionic conditions the current associated with G
s
shows strong outward rectification and its permeability sequence is F− > NO3
− > Cl−. In the same conditions the current associated with G
l
does not rectify and its permeability sequence is F−≫ NO3
−= Cl−. Furthermore, at potentials over +50 mV G
s
, but not G
l
, increases with a time constant of several minutes. Finally the gating of G
s
is affected by stretch of the membrane, which leads to an increased activation and a reduced voltage sensitivity. Anion conductances
similar to the ones described here have been found in many plant preparations but G
l
-type components have been generally interpreted as the background activation of the slow voltage-gated channels (corresponding
to G
s
). We show that in coffee protoplasts G
l
and G
s
are kinetically and biophysically distinct, suggesting that they correspond to two different molecular entities.
Received: 25 November 1996/Revised: 9 April 1997 相似文献