The Myxobacteria

A large number of nonmotile mutants of the gliding bacterium Myxococcus xanthus have been isolated and partly characterized. About [unk] of these mutants are conditional mutants of a novel kind: mutant cells become transiently motile after contact with nonmutant cells or with cells of a different mutant type. These "stimulatable" mutants fall into five phenotypic classes (types B, C, D, E, and F). Most mutants are nonstimulatable (type A) and never become motile, but type A cells (and wild-type cells) can stimulate cells of any of the other five types. Stimulatable mutants of different types are capable of stimulating each other. For example, in a mixture of B and C cells, both become motile. Linkage analysis using a generalized transducing phage has shown that each of types B, C, D, E, and F corresponds to a single distinct genetic locus. Type A mutants, by contrast, belong to at least 17 different loci. Stimulation depends on close apposition of interacting cells, because stimulation does not occur when contact between cells is prevented. It is possible that the stimulatable mutants are defective in components of the gliding mechanism that can be exchanged between cells. Alternatively, they may be defective in a system of cell communication controlling the coordinated cell movements observed in Myxococcus.

Myxobacteria have been shown to produce a peculiar RNA-DNA complex called msDNA, in which a single-stranded DNA is branched out from a RNA molecule (msdRNA) by a 2',5' phosphodiester linkage. It has been predicted that reverse transcriptase is required for msDNA biosynthesis. We identified a gene for reverse transcriptase in M. xanthus in the region that has been demonstrated to code for a cis- or transacting element for msDNA synthesis. This gene is located immediately downstream of the msdRNA coding region, and codes for a polypeptide of 485 amino acid residues. The polypeptide shows sequence similarity with retroviral reverse transcriptases. This fact, together with the mode of msDNA synthesis, suggests a possible relationship between retroviruses and the msDNA system. The analysis of the gene and the distribution of the msDNA system in independent isolates of M. xanthus indicate that the element is as old as other essential genes in M. xanthus and that it was not recently acquired into the genome.