Genetic control of bacterial suicide: regulation of the induction of PBSX in Bacillus subtilis

Abstract

PBSX is a phage-like bacteriocin (phibacin) of Bacilus subtilis 168. Bacteria carrying the PBSX genome are induced by DNA-damaging agents to lyse and produce PBSX particles. The particles cannot propagate the PBSX genome. The particles produced by this suicidal response kill strains nonlysogenic for PBSX. A 5.2-kb region which controls the induction of PBSX has been sequenced. The genes identified include the previously identified repressor gene xre and a positive control factor gene, pcf. Pcf is similar to known sigma factors and acts at the late promoter P which has been located distal to pcf. The first two genes expressed from the late promoter show homology to genes encoding the subunits of phage terminases.The defective bacteriophage PBSX of Bacillus subtilis 168 is a phage-like bacteriocin, or phibacin, with curious biological properties. PBSX lysogens exposed to DNA-damaging agents produce PBSX phage-like particles which kill other nonlysogenic Bacillus strains (25) by binding to and disrupting the cell wall. Each PBSX particle has a small head and a relatively long tail. Phage heads appear to contain randomly selected 13-kb segments of host DNA, which are not injected into susceptible cells. PBSX is therefore not propagated by these particles (25,26). PBSX appears to be a particulate bacteriocin which has evolved from a bacteriophage.Induction of PBSX is controlled by the repressor gene xre (6,27,40,41). The amino acid sequence of Xre predicts that it is a helix-turn-helix (HLH) protein resembling other DNAbinding proteins such as lambda cI and Cro (14, 41). However, little is known about the mechanism of action of Xre, and apart from the suggestion that xre and a neighboring gene, now called open reading frame 10 (ORF10), are regulated by Xre itself, nothing is known about how the late genes are regulated (41).All known structural and lytic protein genes have been shown to be clustered within a large operon of at least 19 kb in length, which appears to be expressed from a single late promoter region called PL (Fig. la) (40). A number of mutations in regulatory genes, including xin (noninducible for PBSX) and xhi (heat inducible for PBSX), are located proximal to mutations affecting phage head and tail proteins such as xhd, xtl, and xki (6, 38). A regulatory mutation, xhi-1479, renders PBSX thermoinducible (6). A 1.2-kb EcoRI fragment from a PBSX wild-type strain was found to complement the xhi-1479 mutation, and sequence analysis identified the xre gene as the site of the mutation (41). xhi-1479 is thus analogous to the cIts857 mutation of bacteriophage lambda (36 Media. B. subtilis and Escherichia coli were grown in LuriaBertani (LB) broth (1% tryptone, 0.5% yeast extract, 0.5% NaCl) or LB agar. For selection, media contained chloramphenicol (at 3 pug/ml for low-copy-number vectors and 5 jig/ml for higher-copy-number vectors) and kanamycin (5 pug/ml) for B. subtilis, or chloramphenicol (15 jig/ml) and ampicillin (100 pug/ml) for E. coli as appropriate. a-Amylase activity was detected by adding starch (...

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

“…That this ancestor was probably a phage is suggested by the similarities of the two elements to phages in terms of their gene organization and by the presence within the elements of genes homologous to phage repressors, terminases and lysis genes (Wood e t a/., 1990a;McDonnell et al, 1994;Midigue et al, 1995;Takemaru e t al., 1995). The highest level of divergence is seen in the early and middle operons, suggesting that the regulation of expression of these two elements may now differ.…”

Red Light Activates the σB-Mediated General Stress Response ofBacillus subtilisvia the Energy Branch of the Upstream Signaling Cascade

“…The only other gene group that was significantly upregulated in response to visible light in stationary phase is a group of 10 genes that are regulated by Xpf, a sigma-like transcription factor involved in the regulation of expression from the prophage PBSX (33). In contrast, the YtvA-dependent light response in logarithmic growth phase resulted in a significant upregulation of the sporulation-specific F regulon, which is normally activated in the prespore during the early stages of sporulation (for a review, see reference 41).…”

Overproduction, isolation, and DNA-binding characteristics of Xre, the repressor protein from the Bacillus subtilis defective prophage PBSX

“…Xre binds to four sites in the region between the divergently transcribed genes xre and ORF10. Xre binding to operators 01 and 02 may repress rightward transcription of ORF10 and other downstream middle genes which are important for lytic growth (3). Xre may regulate transcription of its own gene by binding to 03 and 04, thereby preventing leftward transcription and expression of xre.…”