The origin of mutants

Reverse transcriptase associated with the biosynthesis of the branched RNA-linked msDNA in Myxococcus xanthus

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.

The evolution of ecological tolerance in prokaryotes

The ecological ranges of Archaeobacteria and Eubacteria are constrained by a requirement for liquid water and the physico-chemical stability limits of biomolecules, but within this broad envelope, prokaryotes have evolved adaptations that permit them to tolerate a remarkable spectrum of habitats. Laboratory experiments indicate that prokaryotes can adapt rapidly to novel environmental conditions, yet geological studies suggest early diversification and long-term stasis within the prokaryotic kingdoms. These apparently contradictory perspectives can be reconciled by understanding that, in general, rates and patterns of prokaryotic evolution reflect the developmental history of the Earth's surface environments. Our understanding of modern microbial ecology provides a lens through which our accumulating knowledge of physiology, molecular phylogeny and the Earth's history can be integrated and focussed on the phenomenon of prokaryotic evolution.