Synthesis of DNA in Lactobacillus acidophilus 1~-26 was investigated in the presence of rifampicin which inhibits I~NA and protein synthesis. Increments in DNA of between 140 and 200% were found under these conditions. Indirect evidence is presented that these large increments are not due to the presence of several replication points in the bacterial chromosome at the time of addition of the drug. Incubation with rifampicin was found to result in a progressive decrease in the capacity for DNA synthesis. This decrease was independent of the amount of D~A synthesized during incubation with rifampicin but was dependent on the time of incubation in the presence of the drug. It is proposed that inhibition of protein and RNA synthesis in L. acidophilus does not inhibit initiation of new cycles of chromosome replication but results in a progressive loss of the capacity to replicate DbTA.
It is generally accepted that, in the absence of protein synthesis, DNA synthesis proceeds for some time in bacteria and, after the completion of the running DNA replication cycles, it stops. A 40 % increase in the DNA content is characteristic for such DNA synthesis if all cells of the culture, having one replication point per chromosome, were synthesizing DNA prior to inhibition of protein synthesis. For the initiation of new replication cycles some protein synthesis is required (for a review see Maal~e and Kjeldgaard, 1966).
On the contrary, our earlier studies with Lactobacillus acidophilus R-26 (L. acidophilus 1) demonstrated the possibility that new DbTA replication cycles are initiated even in the absence of protein synthesis, during amino acid starvation.
A 5--10% increase of RNA occurs, however, in L. acidolohilus in the absence of all amino acids 2. Further, a correlation between the extent of RNA and DbTA synthesis during amino acids starvation was observed (So~ka, Koukalov£ and Reich, 1966;Koukalov~ and Reich, 1968; Koukalov£, So~ka and Reich, 1969). Therefore, a possibility existed that the initiation of DNA replication cycles in the absence of amino acids was conditioned by the mentioned RNA synthesis.
Complete inhibition of RNA synthesis was achieved in the present paper by the use of rifampicin. This substance inhibits DNA-dependent-RNA-polymerase, but not the D~qA-dependent-DI~A-polymerase (Hartmann et al., 1967;Wehrli et al., 1968). Secondarily, rifampiein also inhibits protein synthesis as shown by Laneini and Sartori (1968).
In our experiments DNA, R~qA and protein synthesis in L. acidolohilus were followed in the presence of rifampicin. RlqA and protein synthesis were inhibited 1. Abbreviations used: L. acidophilus, Lactobacillus acidophilus B-26; OD, optical density; CPM, impulses per minute; rif, rifampiein.
- Synthetic medium for L. acidophilus contains 18 amino acids; 15 of them are essential (So~ka, 1966).