Identification of acoR, a regulatory gene for the expression of genes essential for acetoin catabolism in Alcaligenes eutrophus H16.

Two hundred thirty-nine base pairs upstream from acoXABC, which encodes the Alcaligenes eutrophus H16 structural genes essential for cleavage of acetoin, the 2,004-bp acoR gene was identified. acoR encodes a protein of 668 amino acids with a molecular mass of 72.9 kDa. The amino acid sequence deduced from acoR exhibited homologies to the primary structures of transcriptional activators such as NifA of Azotobacter vinelandii, NtrC of Klebsiella pneumoniae, and HoxA of A. eutrophus. Striking similarities to the central domain of these proteins and the presence of a typical nucleotide-binding site (GETGSGK) as well as of a C-terminal helix-turn-helix motif as a DNA-binding site were revealed. Between acoR and acoXABC, two different types of sequences with dual rotational symmetry [CAC-(N11 to N18)-GTG and TGT-(N10 to N14)-ACA] were found; these sequences are similar to NtrC and NifA upstream activator sequences, respectively. Determination of the N-terminal amino acid sequence of an acoR'-'lacZ gene fusion identified the translational start of acoR. S1 nuclease protection assay identified the transcriptional start site 109 bp upstream of acoR. The promoter region (TTGCGC-N18-TACATT) resembled the sigma 70 consensus sequence of Escherichia coli. Analysis of an acoR'-'lacZ fusion and primer extension studies revealed that acoR was expressed at a low level under all culture conditions, whereas acoXABC was expressed only in acetoin-grown cells. The insertions of Tn5 in six transposon-induced acetoin-negative mutants of A. eutrophus were mapped within acoR. On the basis of these studies, it is probable that AcoR represents a regulatory protein which is required for sigma 54-dependent transcription of acoXABC.