msDNA of bacteria.

The msDNA-retron element represents the first prokaryotic member of the large and diverse retroelement family found in many eukaryotic genomes (Table II). This prokaryotic retroelement exists as a single copy element in the chromosome of two different bacterial groups: the common soil microbe M. xanthus and the enteric bacterium E. coli. It encodes an RT similar to the polymerases found in retroviruses, containing most of the strictly conserved amino acids found in all RTs. The RT is responsible for the production of an unusual extrachromosomal RNA-DNA molecule known as msDNA. Each composed of a short single strand of RNA and a short single strand of DNA, msDNAs vary considerably in their primary nucleotide sequences, but all share certain secondary structural features, including the unique 2',5' branch linkage that joins the 5' end of the DNA chain to the 2' position of an internal guanosine residue of the RNA strand. It is proposed that msDNA is synthesized by reverse transcription of a precursor RNA transcribed from a region of the retron containing the genes msr (encoding the RNA portion) and msd (encoding the DNA portion) and the ORF (encoding the RT). The precursor RNA transcript folds into a stable secondary structure that serves as both the primer and the template for the synthesis of msDNA. The msDNA-retron elements of E. coli are found in less than 10% of all strains observed, are heterogeneous in nature, and have an atypical aminoacid codon usage for this species, suggesting that this element was transmitted to E. coli by some other source. The presence of directly repeated 26-base-pair sequences flanking the junctions of the Ec67-retron of E. coli also suggests that it may be a mobile element. However, the msDNA-retrons of M. xanthus appear to be as old as other genes native to this species, based on codon-usage data for the RT genes and the fact that every strain of M. xanthus appears to have the same type of msDNA. If the msDNA-retron element originated with the myxobacteria, it would place the existence of retrons before the appearance of eukaryotic cells, suggesting that the bacterial element is perhaps the ancestral gene from which eukaryotic retroviruses and other retroelements evolved.(ABSTRACT TRUNCATED AT 400 WORDS)