Ribosomal RNA genes in the replication origin region of Bacillus subtilis chromosome

Abstract

In Bacillus subtilis, DNA replication proceeds bidirectionally, commencing in the BamHl fragment, B7 (see Fig. 4). The middle EcoRI fragment E19, generated from B7, is the first to be replicated, followed by the E22 fragment in one direction and by the E6′ fragment in the other direction^1,2. The B7 fragment was also shown to inhibit the replication of a plasmid which carried this fragment^3,4 and the minimal essential sequence for the inhibition (sar) was located to a 200-base pair (bp) region of E19, which contains two promoters, as deduced by the sequence analysis⁵. Previous studies showed that the E6′ fragment was unique while E19 and E22 fragments hybridized to multiple sites on the chromosome. In addition, in the course of the analysis of an RNA covalently linked to DNA and synthesized at specific times related to chromosome initiation⁶, we found that this RNA was complementary both to the E19 and E22 fragments and to ribosomal RNA genes⁷. We show here that a ribosomal RNA gene set (rrnO) overlaps the first replicating fragments in one direction. The promoters for these genes are in fact the promoters of the sar sequence.