The replicative origin of the E. coli chromosome binds to cell membranes only when hemimethylated

DNA from the E. coli replicative origin binds with high affinity to outer membrane preparations. Specific binding regions are contained within a 463 bp stretch of origin DNA between positions -46 and +417 on the oriC map. This region of DNA contains an unusually high number of GATC sites, the recognition sequence for the E. coli DNA adenine methylase. We show here that oriC DNA binds to membrane only when it is hemimethylated. The E. coli chromosomal origin is hemimethylated for 8-10 min after initiation of replication, and origin DNA binds to membranes only during this time period. Based on these results, we propose a speculative model for chromosome segregation in E. coli.     

Class II genes of miniature swine

Class II genes of miniature swine have been characterized by restriction fragment length polymorphism (RFLP) analysis and by analysis of a series of clones isolated from a lymphocyte genomic library. For RFLP analysis, DNA samples from three independent major histocompatibility complex homozygous lines and three intra-MHC recombinant lines were digested with a variety of restriction enzymes and analyzed in Southern blots using human cDNA probes for DP, DQ, DR, and DZ alpha genes, and DP, DQ, DR, and DO beta genes. One, or at most two, unique fragments were detected by hybridization with each of the human probes tested. In contrast, multiple bands (five to six for most enzymes examined) were detected by each of the human probes tested, the majority of which were found to cross-react with at least three of these probes under conditions of moderate stringency. Genomic DNA from the SLA ^c haplotype was cloned into an EMBL-3 bacteriophage vector, and the corresponding genomic library was screened with each of these human cDNA probes. The class II genes thereby isolated from this library showed characteristics consistent with those anticipated from the RFLP analysis. Thus, unique genes were obtained which showed no evidence of cross-hybridization, while genes showed extensive cross-hybridization and were frequently detected in the library by more than one human gene probe. These data are consistent with early evolutionary divergence of a genes, prior to mammalian speciation, and with continuing evolution of genes, with possible shared usage of these genes by different a loci. The data also imply that genes can readily be assigned to loci homologous to their human counterparts, but that genes will require further mapping and/or sequence analysis to confirm assignments.     

The effect of food consumption on sodium and water balance in the predaceous diving beetle,Dytiscus verticalis

Summary Sodium and water balance ofDytiscus verticalis in fresh water were investigated under three feeding regimes: unfed, and fed a diet either low or high in sodium chloride. Unfed sodium influx was 0.13 and sodium efflux was 0.74 moles/100 gwm·h. These values are low in comparison with most freshwater animals. The electrical potential difference across the integument in artificial soft water (ASW) was about 150 mV smaller than the potential necessary to maintain sodium balance in the absence of active transport. However, sodium influx did not show saturation kinetics over an external concentration range of 91 to 1725 M. Unfed beetles failed to arrest net sodium loss to baths that were initially distilled water or ASW, even when bath sodium concentrations reached 75–298 M. The long-term rate of net sodium loss ranged from 0.61 to 4.4 moles/100 gwm·h for four sets of animals. Beetles decreased sodium efflux during a period of fasting. During subsequent feeding, beetles fed a high sodium diet (HSD) increased sodium efflux while beetles fed a low sodium diet (LSD) maintained low rates of sodium efflux. HSD fed beetles increased body sodium and hemolymph sodium concentration, and expanded extracellular fluid, relative to LSD fed beetles. Thus beetles cannot achieve sodium balance in fresh water without dietary sodium input, although they are able to regulate sodium loss.Abbreviations gwm grams wet mass - ASW artificial soft water - DW distilled water - HSD high sodium diet - LSD low sodium diet - ECF extracellular fluid volume     

Persistence of community structure: what happens when you change taxonomic scale?

It is common in community ecology to use the species as the taxonomic category of interest, yet in rich tropical assemblages containing guilds of very similar species this may not be appropriate. Such assemblages may be organized at the level of guilds rather than at the finer species level. In a ten-year study of assemblages of fish at One Tree Reef, Great Barrier Reef, we found species composition and the number of fish on a given lagoonal patch reef vary greatly across time (Sale and Douglas 1984; Sale et al. in preparation). The mean average proportional similarity of a reef's assemblage to itself at different times (censuses) is usually low at a value of around 0.5. This apparent variability may be ecologically irrelevant noise if organization is at the higher guild level. We have recast our database at the guild level to test this possibility. Thirteen guilds were defined by the diets, foraging habitats and times of the individual species comprising them. Similarity of an assemblage to itself at successive censuses was re-calculated using the number of individuals in each guild instead of the numbers in each species. This analysis yielded significantly higher levels of similarity (P<0.01) among censuses. To test whether this increase in similarity was due solely to the smaller number of categories used to calculate the similarity indices, 5 sets of randomly generated guilds were constructed using a Monte Carlo approach. No significant difference (P>0.05) was found between the average similarity among censuses when assemblages were classified into these null guilds and when they were classified according to the real guilds. These results indicate that shifting to the larger taxonomic scale of guilds does not reveal a significantly more persistent assemblage structure than that revealed when analysis is at the smaller scale of species. There is no evidence of an underlying organization of these assemblages at the guild level.     

Behavioral,physiological, and morphological components of dominance and mate attraction in male green iguanas

This study investigated the morphological, physiological, and behavioral components of social dominance important for mate attraction in male green iguanas (Iguana iguana). A group of 9 male and 11 female adult green iguanas was studied in a large semi-natural enclosure during one reproductive season (October–January). Four of the nine males never initiated aggressive encounters; the other five were observed to display aggressively toward each other and were ranked in a linear dominance hierarchy. Head size was the most important factor influencing fighting success. Head size and display frequency were positively correlated with plasma testosterone levels. Dominance rank directly influenced ability to monopolize areas containing resources used by females. The quality of a male's home range, measured as his access to a large basking rock in the enclosure, was related to the proportion of potential mates found within his home range. One male greatly surpassed the others in his ability to defend a home range of high quality and attract potential mates. These data suggest that physiological and morphological factors, through their influence on social behavior, may ultimately affect male reproductive fitness. © 1992 Wiley-Liss, Inc.     

Lipid composition of the pea aphid,Acyrthosiphon pisum (Harris) (homoptera: Aphididae), reared on host plant and on artificial media

Polar and neutral lipids and their constitutive fatty acids were quantified in the pea aphid, Acyrthosiphon pisum (Harris), grown on host plant or on a lipid free artificial diet. The results were compared to determine if lipids were involved in the suitability of the diet for continuous rearing of this A. pisum biotype. For apterous adults grown on plants, the lipids were characterized by a low amount of neutral lipids (2.5% weight/fresh weight) almost entirely (96.4%) composed of hexanoyl and sorboyl dimyristin. These storage lipids were higher in the alatae (3.8%), probably correlated with potential flight activity. The phospholipid amounts were identical in these two morphs (1.3–1.4% weight/fresh weight), comprised mainly of phosphatidylethanolamines (52%) and phosphatidylcholines (40.6%). These phospholipids contained a still unidentified fatty acid, with a retention time close to that of linolenic acid and synthesized by the aphid or its bacterial symbionts (not found in plants). The apterous adult aphids reared on an artificial diet showed an accumulation of neutral lipids (8.9% for the first generation); this increase was shown to be slightly greater for the hexanoyl and sorboyl triglycerides. In contrast, the phospholipids decreased in aphids reared on an artificial diet (1.1% and 0.9%, respectively, for first and second generation), correlated with a phospholipid fatty acid profile significantly deficient in C18:3 and in the unidentified peculiar fatty acid. These phospholipids are essential components of biological membranes and a diet-driven deficient synthesis in some of their components may result in the observed symptoms. © 1992 Wiley-Liss, Inc.     

Universal primers for amplification of three non-coding regions of chloroplast DNA

Six primers for the amplification of three non-coding regions of chloroplast DNA via the polymerase chain reaction (PCR) have been designed. In order to find out whether these primers were universal, we used them in an attempt to amplify DNA from various plant species. The primers worked for most species tested including algae, bryophytes, pteridophytes, gymnosperms and angiosperms. The fact that they amplify chloroplast DNA non-coding regions over a wide taxonomic range means that these primers may be used to study the population biology (in supplying markers) and evolution (inter- and probably intraspecific phylogenies) of plants.     

Structure and in vitro replication of DNA templates containing 7,8-dihydro-8-oxoadenine.

Adenine residues in DNA are oxidized under the action of ionizing radiation at the C-8 position to give 7,8-dihydro-8-oxoadenine. The formation of this lesion can be considered a cause of mutations and carcinogenesis. Oligodeoxyribonucleotides 39 and 47 bases long containing a single 7,8-dihydro-8-oxoadenine (8-hydroxyadenine) residue were synthesized by using nucleoside phosphoramidites. They were used as templates to study the copies obtained in vitro by the Klenow fragment and the thermostable Taq DNA polymerase. 7,8-Dihydro-8-oxoadenine does not block the replication and thymine is incorporated opposite the damage. The modifications of the DNA duplex conformation provoked by 7,8-dihydro-8-oxoadenine are minor. 1H-NMR spectroscopy shows that the duplex is in a B form, the sugar in a normal position in the helix and the modified base in the anti position. NMR confirms that 7,8-dihydro-8-oxoadenine exists predominantly in the keto form.     

The solution structure of a DNA hairpin containing a loop of three thymidines determined by nuclear magnetic resonance and molecular mechanics.

We have determined by two-dimensional nuclear magnetic resonance studies and molecular mechanics calculations the three-dimensional solution structure of a 21 residue oligonucleotide capable of forming a hairpin structure with a loop of three thymidine residues. This structure is in equilibrium with a duplex form. At 33 degrees C, low ionic strength and in the presence of MgCl2 the hairpin form dominates in solution. Six Watson-Crick base pairs are formed topped by the loop structure. The residues 1-3 and 18-21 are not complementary and form dangling ends. Distance constraints have been derived from nuclear Overhauser enhancement measurements. These, together with molecular mechanics calculations, have been used to determine the structure. We do not observe stacking of thymidine residues either over the 3' or the 5' end of the stem.