Coordination of expression of Zymomonas mobilis glycolytic and fermentative enzymes: a simple hypothesis based on mRNA stability.

Although Zymomonas mobilis is prototrophic, glycolytic and fermentative enzymes (ethanologenic enzymes) constitute over half of the cytoplasmic protein. In this study, transcript stability, functional message pools, and the abundance of cytoplasmic products were compared for genes encoding eight of these essential enzymes. The transcripts of all were very stable, with half-lives ranging from 8 to 18 min. This transcript stability is proposed as an important feature in Z. mobilis that may distinguish highly expressed genes for energy generation from biosynthetic genes, which are required at much lower levels. The evolution of multiple promoters to enhance transcription from single-copy genes, of structural features that alter translational efficiency, and of differences in protein turnover is hypothesized to serve a subordinate role in the regulation of Z. mobilis gene expression. Among the eight ethanologenic genes examined, differences in transcript stability were found to directly correlate with differences in functional message pools and cytoplasmic protein levels. These differences in transcript stability are hypothesized to have evolved as a primary mechanism to balance the levels of individual enzymes within the glycolytic and fermentative pathways.