Survey of multicopy single-stranded DNAs and reverse transcriptase genes among natural isolates of Myxococcus xanthus

Twenty different isolates of the soil bacterium Myxococcus xanthus were examined for the presence of multicopy single-stranded DNA (msDNA)-producing retroelements, or retrons. Each strain was analyzed by ethidium bromide staining for msDNA, 32P labeling of the msDNA molecule by the reverse transcriptase (RT) extension method, and DNA hybridization experiments with probes derived from two retrons, Mx162 and Mx65, previously cloned from M. xanthus DZF1. These analyses revealed that all M. xanthus strains contain an msDNA very similar to Mx162 msDNA, and 13 strains also contain a second smaller msDNA very similar to Mx65 msDNA. In addition, the strains contained retron-encoded genes msr and msd, which code for msDNA, and a gene for RT responsible for the synthesis of msDNA. These genes show greater than 80% nucleotide sequence similarity to retrons Mx162 or Mx65. The near-ubiquitous occurrence of msDNA retrons among M. xanthus strains and their homogeneous nature are in marked contrast to the highly diverse but rarely occurring msDNA-producing elements of Escherichia coli. The possible origin and evolution of RT and retron elements is discussed in view of these findings.