Germination of Bacillus cereus spores in response to L-alanine and to inosine: the roles of gerL and gerQ operons

Abstract

Bacillus cereus 569 (ATCC 10876) endospores germinate in response to inosine or L-alanine, the most rapid germination response being elicited by a combination of these germinants. The gerI operon has already been characterized as a homologue of the gerA spore-germination receptor family of operons found in all Bacillus spp. examined; the primary defect in gerI mutant spores is in the inosine germination response, although spores were also slower to germinate in L-alanine. Additional transposon-insertion mutants, from similar Tn917-LTV1 mutagenesis and enrichment experiments, now define two more operons, also members of the family of gerA homologues, important in L-alanine and inosine germination. Transposon insertions were identified in an alanine-specific germination locus, named gerL, which represents an operon of three genes, termed gerLA, gerLB and gerLC. By examining the residual germination response to L-alanine in gerI and gerL mutants, it was deduced that the GerL proteins contribute most strongly to the L-alanine germination response, and that the GerI proteins, required primarily in inosine germination, mediate only much slower germination responses to alanine. The L-alanine germination responses mediated by GerL and GerI proteins differ in their germination rates, temperature optima and germinant concentration dependence. The gerQ locus, again identified by transposon insertion, is a second inosine-related germinant-receptor operon. GerQ and GerI proteins are both required for the germination response to inosine as sole germinant, but GerQ has no role in L-alanine germination. Although near-identical homologues of gerI and gerL operons are evident in the Bacillus anthracis genome sequence, there is no evidence of a close homologue of gerQ.