Regulation of nucleic acid synthesis in Lactobacillus acidophillus R-26

Abstract

Lactobacillus acidophillus R-26 has been used to study the regulation of chromosome replication. RNA synthesis in this organism is stringent in that when amino acids are removed, RNA synthesis is drastically reduced. Synthesis can be restored by the addition of chloramphenicol. Actinomycin D inhibits RNA synthesis completely in concentrations above 2 μg/ml. DNA synthesis proceeds in the absence of all amino acids resulting in an overall increase of 40–50 % in the DNA content per cell. This synthesis can be further increased by the addition of either glutamic or aspartic acids. Thymine or deoxyribose starvation results in a 150 % increase in DNA content upon subsequent restoration of these compounds and incubation in the absence of amino acids. This increase can be inhibited by the presence of actinomycin D or chloramphenicol during the period of deoxyriboside starvation. These results are similar to those observed previously for Escherichia coli and have been explained by assuming that: (a) chromosome replication is completed in the absence of all amino acids; (b) thymine or deoxyriboside starvation induces an extra cycle of chromosome replication. It has been found that actinomycin D will inhibit the extra cycle of DNA replication, if present at concentration of 5 μg/ml. This concentration is much higher than that necessary to inhibit RNA synthesis, but much lower than that necessary to inhibit normal DNA synthesis. It is proposed that actinomycin is, in this case, acting directly upon the initiation of DNA replication.