Genes coding for phosphotransacetylase and acetate kinase in Sinorhizobiummeliloti are in an operon that is inducible by phosphate stress and controlled by PhoB

Recent work in this laboratory has shown that the gene coding for acetate k inase (ackA) in Sinorhizobium meliloti is up-regulated in response to phosp hate limitation. Characterization of the region surrounding ackA revealed t hat it is adjacent to pta, which codes for phosphotransacetylase, and that these two genes are part of an operon composed of at least two additional g enes in the following order: an open reading frame (orfA), pfa, ackA, and t he partial sequence of a gene with an inferred peptide that has a high degr ee of homology to enoyl-ACP reductase (fabI), Experiments combining enzyme assays, a chromosomal lacZ::ackA transcriptional fusion, complementation an alysis with cosmid subclones, and the creation of mutations in pta and ackA all indicated that the orfA-pta-ackA-fabI genes are cotranscribed in respo nse to phosphate starvation. Primer extension was used to map the position of the phosphate starvation-inducible transcriptional start sites upstream of orfA. The start sites were found to be preceded by a sequence having sim ilarity to PHO boxes from other phosphate-regulated genes in S, meliloti an d to the consensus PHO box in Escherichia coli, Introduction of a phoB muta tion in the wild-type strain eliminated elevated levels of acetate kinase a nd phosphotransacetylase activities in response to phosphate limitation and also eliminated the phosphate stress-induced upregulation of the ackA::lac Z fusion. Mutations in either ackA alone or both pta and ackA did not affec t the nodulation or nitrogen fixation phenotype of S. meliloti.