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.