YeiL, the third member of the CRP-FNR family in Escherichia coli

The yeiL open reading frame located at 48.5 min (2254 kb) in the nfo-fruA region of the Escherichia coli chromosome was predicted to encode a CRP and FNR paralogue capable of forming inter- or intra-molecular disulphide bonds and incorporating one iron-sulphur centre per 25 kDa subunit, Purified MBP-YeiL (a maltose-binding-protein-YeiL fusion protein) was a high-molecular-mass oligomer or aggregate which released unstable monomers (68 kDa) under reducing conditions. The MBP-YeiL protein contained substoichiometric amounts of iron and acid-labile sulphide, and an average of one disulphide bond per monomer. The iron and sulphide contents increased consistent with the acquisition of one [4Fe-4S] cluster per monomer after anaerobic NifS-catalysed reconstitution. By analogy with FNR and FLP (the FNR-like protein of Lactobacillus casei) it was suggested that the transcription-regulatory activity of YeiL might be modulated by a sensory iron-sulphur cluster and/or by reversible disulphide bond formation. A yeiL-lacZ transcriptional fusion showed that aerobic yeiL expression increases at least sixfold during stationary phase, requires RpoS, and is positively autoregulated by YeiL, positively activated by Lrp (and IHF in the absence of FNR) and negatively regulated by FNR. A regulatory link between the synthesis of YeiK (a potential nucleoside hydrolase) and YeiL was inferred by showing that the yeiK and yeiL genes are divergently transcribed from overlapping promoters. A 10-15% deficiency in aerobic growth yield and an enhanced loss of viability under nitrogen starvation conditions were detected with a yeiL::kan(R) mutant, suggesting that YeiL might function as a post-exponential-phase nitrogen-starvation regulator.