The msDNAs of bacteria

msDNAs are small, structurally unique satellite DNAs found in a number of Gram-negative bacteria. Composed of hundreds of copies of single-stranded DNA-hence the name multicopy single-stranded DNA-msDNA is actually a complex of DNA, RNA, and probably protein. These peculiar molecules are synthesized by a reverse transcription mechanism catalyzed by a reverse transcriptase (RT) that is evolutionarily related to the polymerase found in the HIV virus. The genes, including the RT gene, responsible for the synthesis of msDNA are encoded in a retron, a genetic element that is carried on the bacterial chromosome. The retron is, in fact, the first such retroelement to be discovered in prokaryotic cells. This report is a comprehensive review of the many interesting questions raised by this unique DNA and the fascinating answers it has revealed. We have learned a great deal about the structure of msDNA: how it is synthesized, the structure and functions of the RT protein required to make it, its effects on the host cell, the retron element that encodes it, its possible origins and evolution, and even its potential usefulness as a practical genetic tool. Despite the impressive gains in our understanding of the msDNAs, however, the simple, fundamental question of its natural function remains an enduring mystery. Thus, we have much more to learn about the msDNAs of bacteria.