Characterization of multiple promoters and transcript stability in the sacB-sacC gene cluster in Zymomonas mobilis

In Zymomonas mobilis, the extracellular levansucrase (SacB) and extracellular sucrase (SacC) are involved in sucrose hydrolysis. Genes coding for these two enzymes (sacB and sacC) are arranged in a cluster in the genome and separated by a short intervening sequence. The level of sacC transcript was 12-fold higher than that of sacB transcript. On the other hand, transcript stability analysis in sucrose grown cultures revealed that the half-life of the sacB transcripts (153 s) was more than twofold higher than that of sacC transcript (66 s). The decay curves of sacB and sacC transcripts analyzed by the semi-quantitative RT-PCR correlated well with the decay curves of the respective enzyme activities. In the sacB promoter disruption mutant, Z. moblis BT2, the extracellular sucrase activity decreased from 2.6 to 2.0 U mg(-1) in sucrose medium due to the loss of SacB expression. The expression of sacC in the absence of the sacB promoter suggested that these two genes could be transcribed as different mRNAs. The promoter-lacZ fusion studies in Escherichia coli proved that the short intervening region acts as a strong promoter for the sacC gene.