Phylogenetic distribution of branched RNA-linked multicopy single-stranded DNA among natural isolates of Escherichia coli.

Multicopy single-stranded DNA (msDNA), a branched DNA-RNA molecule, has been shown in Escherichia coli B and clinical strain Cl-1 to be synthesized by reverse transcriptase. We report that 13% of the strains of the ECOR collection, a sample of 72 E. coli isolates representing the breadth of genetic variation of the species, produce msDNA. Three of the four major subspecific groups include msDNA-producing strains. Screening of 25 isolates that are genetically related to msDNA-producing clinical strains uncovered 22 additional msDNA-producing strains. A phylogenetic tree based on allelic variation detected electrophoretically at 20 enzyme-encoding loci revealed two major clusters and several deep branches composed of strains that synthesize msDNA. Although E. coli K-12 does not harbor msDNA, other closely related strains of the K-12 family do. The results support the hypothesis that msDNA-synthesizing systems, including reverse transcriptase genes, were acquired recently and independently in different lineages of E. coli.