The Bacillus subtilis histone-like protein Hbsu is required for DNA resolution and DNA inversion mediated by the beta recombinase of plasmid pSM19035.

The beta recombinase, encoded by the Gram-positive bacterial plasmid pSM19035, is unable to mediate DNA recombination in vitro unless a host factor is provided. The factor has now been identified as the Bacillus subtilis Hbsu protein. Hbsu is a nonspecific DNA-binding and DNA-bending protein. The beta recombinase, in the presence of highly purified Hbsu protein, is able to catalyze in vitro intramolecular recombination between two specific recombination sites on a supercoiled DNA molecule. DNA resolution was obtained when the two crossing over sites (six sites) were directly oriented, whereas DNA inversion was the product when the six sites were in inverse orientation. The ability of the Escherichia coli chromatin-associated proteins HU, IHF, Fis, and H-NS to substitute for Hbsu was investigated. HU efficiently stimulated beta-mediated recombination, while the effect of IHF was partial and that of Fis and H-NS was undetectable. In addition, the beta protein was able to mediate DNA recombination in both wild-type and IHF-deficient E. coli cells, but failed to do so in an HU-deficient strain. The data presented provide direct evidence that a chromatin-associated protein is strictly required for beta-mediated recombination.