The acetylation of the histone-like protein HBsu at specific sites alters gene expression during sporulation in Bacillus subtilis

Sporulation is an adaptive response to starvation in bacteria that consists of a series of developmental changes in cellular morphology and physiology, leading to the formation of a highly resistant endospore. In Bacillus subtilis , there is an intricate developmental program which involves the precise coordination of gene expression and ongoing morphological changes to yield the mature spore. The histone-like protein HBsu is involved in proper spore packaging and compaction of the chromosomal DNA. Previously, we found that the acetylation of different lysine residues on HBsu impairs sporulation frequency and spore resistance properties. One mechanism by which HBsu influences the process of sporulation could be through the regulation of gene expression. To test this idea, we performed RT-qPCR to analyze gene expression throughout the sporulation process in wildtype and seven acetylation-mimicking (glutamine substitutions) mutant strains. Acetylation of HBsu at K41 increased the expression of key early and late sporulation genes, especially during the later stages. For example, overexpression of σ ^F and σ ^G drive expression of their regulon members at inappropriate times. These findings suggest that K41 acetylation activates gene expression and might represent an “on-off” switch for important regulatory factors as cells transition from early to late phases. The gene expression profiles of hbsK3Q , hbsK37Q, hbsK75Q, hbsK80Q, and hbsK86Q mutants were largely unchanged, but did have significant reductions of key late sporulation proteins, which could explain the observed defects in spore resistance properties. We propose that acetylation of HBsu at specific sites directly regulates gene expression during sporulation and this is required for proper timing and coordination.