Retrons are bacterial retroelements encoding the reverse transcriptase as well as msd and msr genes. Retron is organised as an operon and is responsible for msDNA production. Multicopy single-stranded DNA (msDNA) is a structurally unique molecule composed of a single-stranded DNA (encoded by msd) branched out from a single-stranded RNA (encoded by msr) by 2’-5’ phosphodiester linkage. msDNAs are common in cells of soil bacteria (Myxococcus, Stigmatella) but occur also in bacteria from Enterobacteriaceae family. Function of msDNA in bacterial cell is unknown.

A collection of 397 clinical Enterobacteriaceae strains isolated from human and animal hosts was tested. Two different methods were applied.

The search of retron Ec107, the most frequently observed in E. coli, was performed with usage of PCR. Four such strains were identified. The sequence of retron in strains ZMF38 and OM72/DK40 is identical with the sequence from NCBI GenBank database (acc. nr X62583). Strains ML2009 and OM204/DK45b have point mutations; in strain ML2009, a substitution causing conservative change of aminoacids is present.

The new method of msDNA detection by direct analysis in polyacrylamide gel was developed. It consists of total DNA preparation and PAGE, band appears after ethidium bromide or silver staining. Five E. coli strains possessing msDNA were discovered. For two strains single stranded DNA purified from polyacrylamide gel was sequenced by Maxam-Gilbert method. Sequence obtained for E. coli CDZ 264 strain was the same as previously published retron Ec86 (NCBI acc nr X6026).

Strain CZD1527 showed unique sequence of msDNA and was analysed in details. Whole retron sequence was obtained, it is 3601 bp long and was published in NCBI GenBank acc. no EF428983). It is a new retron showing very limited homology on nucleotide level to Vibrio retrons and E. coli Ec110 retron.

Retron contains msd, msr genes, reverse transcriptase gene and additional open reading frame named hmp. The protein encoded by hmp exhibits same homology to membrane proteins. The reverse transcriptase exhibits app. 40% homology to bacterial the reverse transcriptases previously described.

All the retron strains isolated in this study turned out to be E. coli. The applied method of direct detection of msDNA in polyacrylamide gels proved to be effective in case of E. coli strains.

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