The phage-like element PBSX and part of the skin element, which are resident at different locations on the Bacillus subtilis chromosome, are highly homologous.

PBSX and skin are two unusual genetic elements resident on the Bacillus subtilis chromosome. PBSX is a phage-like element located at approximately 100 degrees which is induced by the SOS response and results in cell lysis with the release of phage-like particles. The phage particles contain bacterial chromosomal DNA and kill sensitive bacteria without injecting DNA. The skin element is located at approximately 230 degrees on the chromosome and is positioned within the sigK open reading frame (ORF). It is excised at a particular stage of sporulation, leading to reconstitution of the complete sigK gene. In this paper, we show that there are phage-like operons present in the skin element which are highly homologous to the region of PBSX comprising part of the control region and the late operon. These operons are similar in terms of their gene organization, the percentage identity of the products of homologous ORFs and the positioning and strengths of ribosome-binding sites for each ORF. Although this high degree of conservation suggests that the phange-like operons in skin can be expressed, expression of the late operon was not detected during exponential growth, during sporulation or after induction of the SOS response. However two non-phage-like operons in the skin element are expressed and have distinct expression profiles that are dependent on the growth and developmental status of the cell.