The NO-responsive hemoglobins of Campylobacter jejuni: Concerted responses of two globins to NO and evidence in vitro for globin regulation by the transcription factor NssR.

Campylobacter jejuni possesses NO-responsive and -detoxifying mechanisms to survive NO during transmission and pathogenesis. C. jejuni possesses two hemoglobins. The first (Cgb) is a single-domain (non-flavo)hemoglobin encoded by gene Cj1586 (cgb), mutation of which leads to hypersensitivity to S-nitrosoglutathione and NO. Transcription of cgb is induced by nitrosative stress and confers resistance to NO, presumably via a Cgb-catalyzed dioxygenase or denitrosylase reaction that converts NO and oxygen to nitrate. Expression of Cgb in response to NO is mediated via the positively-acting transcription factor NssR, which regulates expression of a small regulon that includes cgb and ctb (Cj0465c), the latter encoding the truncated hemoglobin, Ctb. The role of Ctb is unclear: it is not directly involved in NO detoxification but is implicated in oxygen delivery or metabolism. Here, we describe attempts to define a function for Ctb by examining the effects of a ctb mutation on the NO transcriptome and cgb gene expression during normoxia and hypoxia. Mutation of ctb does not elicit major compensatory transcriptomic changes but relatively minor changes in genes involved in intermediary metabolism, solute transport and signal transduction. We present and test the hypothesis that, by binding NO or O(2), Ctb dampens the response to NO under hypoxic conditions and limits cgb expression, perhaps because Cgb function (i.e. NO detoxification) requires O(2)-dependent chemistry. We report the purification of NssR and specific binding to the ctb promoter. GSNO does not affect the high affinity of NssR for the ctb promoter.