msDNA of Bacteria

This chapter describes the molecular mechanisms of msDNA biosynthesis, its functions, and the evolutionary implications of the retron element. The msDNA molecule consists of a short single strand of DNA joined, at its 5’ end, to a nucleoside in the middle of a single strand of RNA; specifically the 5’ end of the DNA chain is joined to the 2’ position of an internal uanosine residue of the RNA molecule, forming a unique 2’, 5’ phosphodiester bond between RNA and DNA. This unusual structure is a hallmark of the abundant, but functionally obscure, msDNA and remains perhaps the most peculiar of many unusual features of the msDNA system. Because the msDNA-synthesizing systems of bacteria encode RT, but to the present knowledge, lack some of the other features of retroviruses, such as long terminal repeats and virions; the systems have been termed “retrons”. The msDNA molecule probably exists free of the chromosome in the cell cytoplasm, and can be isolated by the same methods used to isolate plasmids.