The global regulators GacA and sigma s form part of a cascade that controls alginate production in Azotobacter vinelandii

Transcription of the Azotobacter vinelandii algD gene, which encodes GDP-ma nnose dehydrogenase (the rate-limiting enzyme of alginate synthesis), start s from three sites: p1, p2, and p3. The sensor kinase GacS, a member of the two-component regulatory system, is required for transcription of algD fro m its three sites during the stationary phase. Here we show that algD is ex pressed constitutively throughout the growth cycle from the p2 and p3 sites and that transcription from p1 started at the transition between the expon ential growth phase and stationary phase. We constructed A. vinelandii stra ins that carried mutations in gacA encoding the cognate response regulator of acS and in rpoS coding for the stationary-phase sigma (s) factor. The ga cA mutation impaired alginate production and transcription of algD from its three promoters. Transcription of rpoS was also abolished by the gacA muta tion. The rpoS mutation impaired transcription of algD from the pl promoter and increased it from the p2 sigma (E) promoter. The results of this study provide evidence for the predominant role of GacA in a regulatory cascade controlling alginate production and gene expression during the stationary p hase in A. vinelandii.