Regulatory control of temporally expressed integration host factor (IHF) in Legionella pneumophila

Legionella pneumophila, an intracellular parasite of protozoa, possesses a distinct dimorphic life cycle that alternates between the vegetative replicative form and the resilient but highly infectious cyst form. Previously, temporally expressed heterodimeric integration host factor (IHF) was shown to be required for differentiation into the cyst form. However, the precise regulatory mechanisms controlling the expression of IHF have not been identified. Microplate kinetic assays with GFP reporter promoter fusion constructs in wild-type, Δihf, ΔrpoS and ΔletA mutant strain backgrounds were employed to assess differences in expression levels of ihfA, ihfB, rsmY and rsmZ. Loss of IHF, RsmY and RsmZ expression in various mutant strain backgrounds was confirmed by quantitative PCR. Here we report that the stationary phase sigma factor RpoS is a positive regulator of IHF, whereas IHF appears to act as a positive autoregulator assisting RpoS. Bioinformatic analyses identified a set of IHF binding sites upstream of one RpoS binding site in the promoter region for both ihfA and ihfB. Recombinant IHF protein bound ihfA and ihfB promoter regions in vitro, confirming the functionality of these IHF binding sites that may assist in the bending of the promoter DNA to facilitate transcription activation of ihfA and ihfB by RpoS. Interestingly, the consensus binding site for IHF is very similar to that of the two-component response regulator LetA. LetA negatively regulates transcription of ihfA and ihfB, implying titrational regulatory control by LetA and IHF. Along with LetA, IHF was found to positively regulate expression of the non-coding regulatory RNAs RsmY and RsmZ responsible for the de-repression of CsrA-repressed transcripts associated with cyst formation, and coordinated post-exponential virulent phenotypes. Taken together, these observations indicate that IHF may have more of an integral role in the global regulatory system governing the transition from replicative to cyst forms than previously thought.