Possible involvement of DNA-linked RNA in the initiation of Bacillus subtilis chromosome replication

An RNA-DNA copolymer whose synthesis is correlated with the transcriptional requirement for chromosomal initiation in Bacillus subtilis contains ribosomal RNA sequences

During synchronous replication induced in a temperature-sensitive initiation mutant of Bacillus subtilis, we previously isolated an RNA covalently linked to DNA. This molecule was synthesized at specific times, correlated with the period of transcription that is required to initiate a new round of replication. In this paper, we show that both RNA and DNA components of the RNA-DNA molecule hybridized with the coding strand for ribosomal RNA. Competition hybridization experiments also demonstrated that ribosomal RNA sequences represent the great majority of the RNA that is linked to DNA. Both RNA and DNA components of the RNA-DNA molecule also hybridized with a region close to the origin of replication, in particular with E19 and E22, two restriction fragments that are replicated early. These fragments in fact form part of the ribosomal operon rrnO. The role of the RNA-linked DNA molecule in initiation in B. subtilis and the possibility that this molecule emanates directly from rrnO rather than from other ribosomal RNA genes are discussed.

Chromosomal initiation in Bacillus subtilis may involve two closely linked origins

When the dnaB37 initiation mutant of Bacillus subtilis is returned to a permissive temperature following a period at 45 degrees C, a synchronous round of DNA replication immediately ensues. Using this system we have been able to analyse the first fragments to be replicated while avoiding the use of thymine starvation or inhibitors of DNA replication. Such treatments are necessary to achieve even modest synchrony in germinating spores. Our results showed that the first fragment to be replicated was a 4 kb BamHI-SalI restriction fragment, BS6. In contrast, when the analysis was performed out in the presence of novobiocin, an inhibitor of DNA gyrase, replication from BS6 was inhibited and the first fragment to be replicated was BS5, a 5.6 kb fragment located 1.7 kb to the right of BS6. Replication from both putative origins was suppressed by rifamycin and was dependent upon dnaB. The results are discussed in relation to previous attempts to identify the first replicating fragment in germinating spores. We also discuss the possibility that B. subtilis contains two origins and suggest that either can act as the primary origin under certain conditions, or alternatively that both origins may act in concert in normal bidirectional replication, each site being required for the leading strand in each direction.

Bacterial growth and division: genes, structures, forces, and clocks

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