The L egionella pneumophila kai operon is implicated in stress response and confers fitness in competitive environments

Abstract

Summary L egionella pneumophila uses aquatic protozoa as replication niche and protection from harsh environments. Although L . pneumophila is not known to have a circadian clock, it encodes homologues of the KaiBC proteins of C yanobacteria that regulate circadian gene expression. We show that L . pneumophila kaiB , kaiC and the downstream gene lpp1114 , are transcribed as a unit under the control of the stress sigma factor RpoS . KaiC and KaiB of L . pneumophila do not interact as evidenced by yeast and bacterial two‐hybrid analyses. Fusion of the C ‐terminal residues of cyanobacterial KaiB to L egionella KaiB restores their interaction. In contrast, KaiC of L . pneumophila conserved autophosphorylation activity, but KaiB does not trigger the dephosphorylation of KaiC like in C yanobacteria . The crystal structure of L . pneumophila KaiB suggests that it is an oxidoreductase‐like protein with a typical thioredoxin fold. Indeed, mutant analyses revealed that the kai operon‐encoded proteins increase fitness of L . pneumophila in competitive environments, and confer higher resistance to oxidative and sodium stress. The phylogenetic analysis indicates that L . pneumophila KaiBC resemble S ynechosystis KaiC2B2 and not circadian KaiB1C1 . Thus, the L . pneumophila K ai proteins do not encode a circadian clock, but enhance stress resistance and adaption to changes in the environments.

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