The Zymomonas mobilis glf, zwf, edd, and glk genes form an operon: localization of the promoter and identification of a conserved sequence in the regulatory region

The Zymomonas mobilis genes that encode the glucose-facilitated diffusion transporter (glf), glucose-6-phosphate dehydrogenase (zwf), 6-phosphogluconate dehydratase (edd), and glucokinase (glk) are clustered on the genome. The data presented here firmly establish that the glf, zwf, edd, and glk genes form an operon, in that order. The four genes of the operon are cotranscribed on a 6.14-kb mRNA. The site of transcriptional initiation for the polycistronic message was mapped by primer extension and nuclease S1 protection analysis. The glf operon promoter region showed significant homology to other highly expressed Z. mobilis promoters, but not to consensus promoters from other bacteria. The highly expressed Z. mobilis promoter set contains two independent, overlapping, conserved sequences that extend from approximately bp -100 to +15 with respect to the transcriptional start sites. Expression of the glf operon was shown to be subject to carbon source-dependent regulation. The mRNA level was threefold higher in cells grown on fructose than in cells grown on glucose. This increase was not the result of differential mRNA processing when cells were grown on the different carbon sources, nor was it the result of differential transcript stability. Degradation of the 6.14-kb glf operon mRNA was biphasic, with initial half-lives of 11.5 min in fructose-grown cells and 12.0 min in glucose-grown cells. Thus, the higher level of glf operon mRNA in fructose-grown cells is the result of an increased rate of transcription. The importance of increasing glf expression in cells growing on fructose is discussed.