ihfA gene of the bacterium Myxococcus xanthus and its role in activation of carotenoid genes by blue light

Myxococcus xanthus responds to blue light by producing carotenoids. Several regulatory genes are known that participate in the light action mechanism, which leads to the transcriptional activation of the carotenoid genes. We had already reported the isolation of a carotenoid-less, Tn5-induced strain (MR508), whose mutant site was unlinked to the indicated regulatory genes, Here, we show that Omega MR508::Tn5 affects all known light-inducible prom oters in different ways. It blocks the activation of two of them by light b ut makes the activity of a third one light independent. The Omega MR508 loc us has been cloned and sequenced. The mutation had occurred at the promoter of a gene we propose is the M. xanthus ortholog of ihfA. This encodes the or subunit of the histone-like integration host factor protein. An in-frame deletion within ihfA causes the same effects as the Omega MR508::Tn5 inser tion. Like other IhfA proteins, the deduced amino acid sequence of M. xanth us IhfA shows much similarity to HU, another histone-like protein. Sequence comparison data, however, and the finding that the M. xanthus gene is prec eded by gene pheT, as happens in other gram-negative bacteria, strongly arg ue for the proposed orthology relationship. The M. xanthus ihfA gene shows some unusual features, both from structural and physiological points of vie w. In particular, the protein is predicted to have a unique, long acidic ex tension at the carboxyl terminus, and it appears to be necessary for normal cell growth and even vital for a certain wild-type strain of M. xanthus.