Determinants of an unusually stable mRNA in the bacterium Myxococcus xanthus.

Myxococcus xanthus is a Gram-negative bacterium which has a complex life cycle that includes development (fruiting body formation). The gene for myxobacterial haemagglutinin, mbhA, is developmentally regulated and highly expressed. In this report we show that the mbhA mRNA is exceptionally stable for a prokaryotic organism, exhibiting a chemical half life (t1/2) of 150 min at 18 h of development. The mbhA mRNA was not stable in vegetatively growing cells nor was it stable when expressed in Escherichia coli. We have used site-directed mutagenesis of the mbhA gene to analyse some of the determinants which mediate the stability of the mbhA transcript. Sequences within the 3'-untranslated region (3'-UTR) were found to be crucial for mRNA stability. This region of mRNA can potentially form an extremely stable stem-loop structure immediately adjacent to the translational stop codon. A deletion within this region caused a 10-fold increase in the decay rate of the transcript. Furthermore, conditions which were associated with reduced mbhA translation or mutations that caused premature termination of translation drastically reduced mRNA stability even in the presence of the wild type 3'-UTR. These results suggest that a significant aspect of mbhA mRNA stability involves a synergistic interaction of the translational machinery with sequence elements within the 3'-UTR.