The Bacillus subtilis HBsu protein modifies the effects of alpha/beta-type, small acid-soluble spore proteins on DNA

HBsu, the Bacillus subtilis homolog of the Escherichia coli HU proteins and the major chromosomal protein in vegetative cells of B, subtilis, is prese nt at similar levels in vegetative cells and spores (similar to 5 x 10(4) m onomers/genome). The level of HBsu in spores was unaffected by the presence or absence of the alpha/beta-type, small acid-soluble proteins (SASP), whi ch are the major chromosomal proteins in spores. In developing forespores, HBsu colocalized with alpha/beta-type SASP on the nucleoid, suggesting that HBsu could modulate alpha/beta-type SASP-mediated properties of spore DNA, Indeed, in vitro studies showed that HBsu altered alpha/beta-type SASP pro tection of pUC19 from DNase digestion, induced negative DNA supercoiling op posing alpha/beta-type SASP-mediated positive supercoiling, and greatly ame liorated the alpha/beta-type SASP-mediated increase in DNA persistence leng th. However, HBsu did not significantly interfere with the alpha/beta-type SASP-mediated changes in the UV photochemistry of DNA that explain the heig htened resistance of spores to UV radiation. These data strongly support a role for HBsu in modulating the effects of alpha/beta-type SASP on the prop erties of DNA in the developing and dormant spore.