The ripX locus of Bacillus subtilis encodes a site-specific recombinase involved in proper chromosome partitioning

The Bacillus subtilis ripX gene encodes a protein that has 37 and 44% ident ity with the XerC and XerD site-specific recombinases of Escherichia coil. XerC and XerD are hypothesized to act in concert at the dif site to resolve dimeric chromosomes formed by recombination during replication. Cultures o f ripX mutants contained a subpopulation of unequal-size cells held togethe r in long chains. The chains included anucleate cells and cells with aberra ntly dense or diffuse nucleoids, indicating a chromosome partitioning failu re. This result is consistent with RipX having a role in the resolution of chromosome dimers in B. subtilis. Spores contain a single uninitiated chrom osome, and analysis of germinated, outgrowing spores showed that the placem ent of FtsZ rings and septa is affected in ripX strains by the first divisi on after the initiation of germination. The introduction of a recA mutation into ripX strains resulted in only slight modifications of the ripX phenot ype, suggesting that chromosome dimers can form in a RecA-independent manne r in B. subtilis. In addition do RipX, the CodV protein of B. subtilis show s extensive similarity to XerC and XerD. The RipX and CodV proteins were sh own to bind in vitro to DNA containing the E. coli dif site. Together they functioned efficiently in vitro to catalyze site-specific cleavage of an ar tificial Holliday junction containing a dif site. Inactivation of codV alon e did not cause a discernible change in phenotype, and it is speculated tha t RipX can substitute for CodV in vivo.