Involvement of the FtsH (HflB) protease in the activity of sigma(54) promoters

The effect of FtsH, an essential inner membrane-bound protease, in the regu lation of the sigma(54)-dependent Pu promoter has been examined in vivo. Es cherichia coli cells lacking FtsH failed to activate a Pu-lacZ fusion in re sponse to the cognate enhancer-binding protein XylR. However, the intracell ular concentrations of XylR and sigma(54), as well as their apparent physic al integrity were the same regardless of the presence or absence of the pro tease. The loss of Pu activity in FtsH-minus cells was not due to the imbal ance between sigma factors caused by the lack of the protease. ftsH mutants could not grow in media with glutamine as the only nitrogen source and fai led also to induce the sigma(54) promoters PnifH by NifA and PpspA by PspF. These lesions were fully complemented by a ftsH(+) plasmid. Therefore, par t of the pleiotropic phenotype of FtsH-less cells corresponded to the lack of sigma(54) activity. Overproduction of sigma(54), however, restored both transcriptional activity of Pu and growth in glutamine of a ftsH strain. Th ese observations suggested that the activity of sigma(54) is checked in viv o by an interplay of factors that ultimately determine the performance of c ognate promoters under given physiological conditions.