The stringent response, σH-dependent gene expression and sporulation in Bacillus subtilis

Recently, we found that transcription of the yvyD gene of Bacillus subtilis from a sigma (H)-dependent promoter is induced in response to amino acid starvation. This induction occurred in the wild type, but not in the relA mutant, suggesting that ppGpp might be involved in the activa tion of sigma (H). In order to substantiate this finding we looked for other sigma (H)-dependent genes which also required an active relA gene product for activation in amino acid-starved cells. Three additional transcription units, namely spo0A, spoVG and the yt-xGHI operon (also known as the csb40 operon), were found to be under the dual control of RelA and BH in amino acid-starved cells, but many other sigma (H)-dependent genes were not activated under these conditions. Of particular interest is the observation that spo0A is one of the genes that need a functional relA gene product for sigma (H)-dependent gene activation. Furthermore, in sporulation medium, both Spo0A accumulation and phosphorylation are delayed in the relA deletion strain, leading to a delay in sporulation. A comparison between a relA point mutant and a relA deletion mutant revealed that the relA deletion mutant exhibits a more pronounced delay in the appearance of spores. These results suggest that the stringent response is not required for sporulation per se, but rather enhances the efficiency of sporulation.