Genetic and base sequence homologies in bacillus.

influences are negligible, and that the changing environment and the nature of the gene product primarily determine those genes which evolve most rapidly. On the other hand, map position influences on evolutionary rate could be of such great importance that they provide selection pressure for map rearrangement. Because the genome of Bacillus subtilis is replicated in sequential order from one end to the other (YOSHIKAWA and SUEOKA 1963 a, b), the replicating genome in growing cells has an average of two (or sometimes more (OISHI, YOSHIKAWA and SUEOKA 1964) ) copies of early-replicating genes per copy of late-replicating ones. Unless genes mutate only at replication, this gene dosage effect could impose a pattern on the rate of evolution of Bacillus genes, since the target size for mutation of early genes is effectively doubled. Moreover, genes for which high dosage is advantageous might be localized in the early-replicating portion of the genome. DUBNAU et al. (1965) have pointed out that the position of rRNA and sRNA genes, the major cluster of which is early-replicating (OISHI and SUEOKA 1965; DUBNAU, SMITH and MARMUR 1965; DUBNAU et al. 1965; OISHI, OISHI and SUEOKA 1966; SMITH et al. 1968), can be rationalized in this way. Structural features such as membrane association of the origin and terminus of the B. subtilis genome (SUEOKA and QUINN 1968) might also influence the rate of evolution of genes near the origin and terminus. The genus Bacillus offers unique advantages for a study of apparent evolution rate us. map position because a transformation system and detailed genetic map are available. Heterologous transformation efficiency can be used as an index of genetic divergence. Further, DNA-DNA heteroduplexes may be prepared from strands originating from two different strains. This makes possible the measurement of the temperature of irreversible strand separation (Tm,,) for various markers in heteroduplex DNA. The mdting point depression, AT,,, (compared to homologous duplexes) is a measure of the percentage of mismatched bases in the most GC-rich portion of the heteroduplex. Since AT,,, estimates correlate well with AT,, estimates (optical melting curves) of percentage of mismatched bases (LAIRD, MCCONAUGHY and MCCARTHY 1969), the AT,,^ values can be taken as a measure of genetic divergence for the entire DNA heteroduplex bearing the marker. T? ase sequence has not been examined in detail. I t is possible that map position