Properties of Bacillus subtilis small, acid-soluble spore proteins with changes in the sequence recognized by their specific protease

Abstract

α/β-type small, acid-soluble proteins (SASP) of dormant spores of Bacillus subtilis bind to DNA and increase its resistance to a variety of damaging agents both in vivo and in vitro. When spores germinate, degradation of α/β-type SASP is rapidly initiated by a sequence-specific protease, which is termed GPR. Three mutations have been introduced into the B. subtilis sspC gene, which codes for the wild-type α/β-type SASP SspC(wt); all three mutations change residues in the highly conserved sequence recognized by GPR. In one mutant protein (SspC(V)), residue 33 (Ser) was changed to Val; in the second (SspC(DL)), residues 30 and 31 (Glu and Ile) were changed to Asp and Leu, respectively; and in the third mutant protein (SspC(DLV)), residues 30, 31, and 33 were changed to Asp, Leu, and Val. All three mutant proteins were rapidly degraded by GPR during spore germination, and SspC(DL) and SspC(DLV) were degraded by GPR in vitro at rates 8 to 9% of that for SspC(wt), although not exclusively at the single site cleaved by GPR in SspC(wt). These results indicate (i) that the sequence specificity of GPR is broader than originally imagined and (ii) that GPR can cleave the sequence in SspC(DLV). Since the latter sequence is identical to that cleaved during the proteolytic activation of GPR, this result further supports an autoprocessing model for GPR activation during sporulation. The properties of these mutant proteins were also examined, both in vivo in B. subtilis spores and in Escherichia coli and in vitro with purified protein. SspC(V) interacted with DNA similarly to SspC(wt) in vivo, restoring UV and heat resistance to spores lacking major α/β-type SASP to the same extent as SspC(wt). In contrast, SspC(DL) had much less effect on DNA properties in vivo and bound strongly only to poly(dG)·poly(dC) in vitro; SspC(DLV) exhibited only weak binding to poly(dG)·poly(dC) in vitro. These results confirm the importance of the conserved primary sequence of α/β-type SASP in the binding of these proteins to spore DNA and alteration of DNA properties and show further that the GPR recognition region in α/β-type SASP plays some role in DNA binding.