N-terminal amino acid residues mediate protein-protein interactions between DNA-bound alpha /beta -type small,...

The binding of alpha/beta-type small, acid-soluble spore proteins (SASP) to DNA of spores of Bacillus species is the major determinant of DNA resistance to a variety of damaging treatments. The primary sequence of alpha/beta-type SASP is highly conserved; however, the N-terminal third of these proteins is less well conserved than the C-terminal two-thirds. To determine the functional importance of residues in the N-terminal region of alpha/beta-type SASP, variants of SspC (a minor alpha/beta-type SASP from Bacillus subtilis) with modified N termini were generated and their structural and DNA binding properties studied in vitro and in vivo. SspC variants with deletions of up to 14 residues ( approximately 20% of SspC residues) were able to bind DNA in vitro and adopted similar conformations when bound to DNA, as determined by circular dichroism spectroscopy and protein-protein cross-linking. Progressive deletion of up to 11 N-terminal residues resulted in proteins with progressively lower DNA binding affinity. However, SspC(Delta)(14) (in which 14 N-terminal residues have been deleted) showed significantly higher affinity for DNA than the larger proteins, SspC(Delta)(10) and SspC(Delta)(11). The affinity of these proteins for DNA was shown to be largely dependent upon the charge of the first few N-terminal residues. These results are interpreted in the context of a model for DNA-dependent alpha/beta-type SASP protein-protein interaction involving the N-terminal regions of these proteins.