The role of DNA replication and isochores in generating mutation and silent substitution rate variance in mammals

SummaryIt has been suggested that isochores are maintained by mutation biases, and that this leads to variation in the rate of mutation across the genome. A model of DNA replication is presented in which the probabilities of misincorporation and proofreading are affected by the composition and concentration of the free nucleotide pools. The relationship between sequence G + C content and the mutation rate is investigated. It is found that there is very little variation in the mutation rate between sequences of different G + C contents if the total concentration of the free nucleotides remains constant. However, variation in the mutation rate can be arbitrarily large if some mismatches are proofread and the total concentration of free nucleotides varies. Hence the model suggests that the maintenance of isochores by the replication of DNA in free nucleotide pools of biased composition does not lead per se to mutation rate variance. However, it is possible that changes in composition could be accompanied by changes in concentration, thus generating mutation rate variance. Furthermore, there is the possibility that germ-line selection could lead to alterations in the overall free nucleotide concentration through the cell cycle. These findings are discussed with reference to the variance in mammalian silent substitution rates.

31P nuclear magnetic resonance study of trout spermatozoa at rest, after motility, and during short-term storage

31P nuclear magnetic resonance was utilized to study respiration in spermatozoa isolated from the rainbow trout. The spectrum obtained under aerobic conditions revealed the presence of phosphomonoesters, intracellular inorganic phosphate, phosphodiesters, phosphocreatine, and free nucleotide triphosphates. The anaerobic spectrum was characterized by an increase in the inorganic phosphate resonance and by the disappearance of the phosphagen and free nucleotide triphosphate peaks. Upon reaeration, the aerobic spectrum was once again observed. Initiation of motility using fish Ringer or freshwater resulted in a decrease in phosphocreatine and nucleotide di- and tri-phosphate levels along with an increase in inorganic phosphate. The presence or absence of a phosphagen peak seemed to be correlated with motility. Preliminary studies on short-term storage of spermatozoa were also performed. These findings are analyzed in terms of the maintenance of motility in the spermatozoa of fish and some invertebrates.

The effects of magnesium ions on the interactions of ox brain and liver glutamate dehydrogenase with ATP and GTP

The effects of ATP and GTP on the activities of ox liver and brain glutamate dehydrogenase were determined in the absence and presence of added Mg2+ ions. Although GTP was an inhibitor of the enzyme reaction assayed in the direction of NAD+ reduction, the magnesium complex of this nucleotide had no effect on the activity. Similarly the magnesium complex of ATP was without effect on the activity of the enzyme although the free nucleotide was an activator. These results suggest that it is important to take account of magnesium complex formation when considering the regulatory actions of these nucleotides.