Homocysteine thiolactone is a positive effector of σSlevels inEscherichia coli

sigma(S) is a regulator of the stationary phase response in Escherichia coli. Multi-copy suppressors were sought in a strain with decreased levels of sigma(S) and one such suppressor was found to encode HsrA, a putative efflux pump. Multi-copy expression of hsrA was shown to lead to accumulation of homocysteine, which is predicted to cause an increase in homocysteine thiolactone (HCTL) levels. A direct correlation between HCTL levels and sigma(S) accumulation was observed both in mutants and during normal cell growth, leading to the hypothesis that HCTL is a physiologically relevant positive effector of sigma(S) levels in vivo.