Myxococcus xanthus synthesizes a stabilized messenger RNA during fruiting body formation

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.

Control of morphogenesis in myxobacteria

The myxobacteria are Gram-negative soil bacteria that live in large communities known as swarms. The most remarkable characteristic of myxobacteria is their ability to form fruiting bodies that have a species-specific shape and color. Fruiting body formation requires the concerted effort of hundreds of thousands of cells. Development is initiated only when two conditions are satisfied. The cells must be nutritionally deprived (environmental signal) and there must be many other cells in the vicinity (intercellular signal). The development of one species, Myxococcus xanthus, has been studied in the most detail. M. xanthus uses amino acids as its primary carbon, nitrogen, and energy source. Starvation for a single amino acid, or for inorganic phosphate, serves as the environmental signal. A variety of intercellular signals appear to control the initiation of development and the timing of subsequent developmental events.

Cell interactions in myxobacterial growth and development

During their complex life cycle, myxobacteria manifest a number of cell interactions. These include contact-mediated interactions as well as those mediated by soluble extracellular signals. Some of these interactions are well-defined; in addition, the tools for molecular and genetic analysis of these interactions in Myxococcus xanthus are now available.

Gene expression during development of Myxococcus xanthus. Analysis of the genes for protein S

Protein S is an abundant spore coat protein produced during fruiting body formation (development) of the bacterium Myxococcus xanthus. We have cloned the DNA which codes for protein S and have found that this DNA hybridizes to three protein S RNA species from developmental cells but does not hybridize to RNA from vegetative cells. The half-life of protein S RNA was found to be unusually long, about 38 minutes, which, at least in part, accounts for the high level of protein S synthesis observed during development. Hybridization of restriction fragments from cloned M. xanthus DNA to the developmental RNAs enabled us to show that M. xanthus has two directly repeated genes for protein S (gene 1 and gene 2) which are separated by about 10(3) base-pairs on the bacterial chromosome. To study the expression of the protein S genes in M. xanthus, eight M. xanthus strains were isolated with Tn5 insertions at various positions in the DNA which codes for protein S. The strains which contained insertions in gene 1 or between gene 1 and gene 2 synthesized all three protein S RNA species and exhibited normal levels of protein S on spores. In contrast, M. xanthus strains exhibited normal levels of protein S on spores. In contrast, M. xanthus strains with insertions in gene 2 had no detectable protein S on spores and lacked protein S RNA. Thus, gene 2 is responsible for most if not all of the production of protein S during M. xanthus development. M. xanthus strains containing insertions in gene 1, gene 2 or both genes, were found to aggregate and sporulate normally even though strains bearing insertions in gene 2 contained no detectable protein S. We examined the expression of gene 1 in more detail by constructing a fusion between the lacZ gene of Escherichia coli and the N-terminal portion of protein S gene 1 of M. xanthus. The expression of beta-galactosidase activity in an M. xanthus strain containing the gene fusion was shown to be under developmental control. This result suggests that gene 1 is also expressed during development although apparently at a much lower level than gene 2.