Multicopy single-stranded DNA (msDNA) is a type of extrachromosomal satellite DNA which consists of a single-stranded DNA molecule linked via a phosphodiester bond to a branched RNA molecule. The resultant structure possesses two stem-loop structures, and functions as both a primer and a template for the creation of more msDNA. It is known to encode a type of reverse transcriptase. Its function remains unknown even though many copies are present within cells.
Before the discovery of msDNA in Myxoccocus xanthus, a species of soil-dwelling bacteria, it was thought that the enzymes known as reverse transcriptases (RT) existed only in eukaryotes and viruses. The discovery led to an increase in research of the area, subsequently msDNA was found to exist within certain strains of the bacterium Escherichia coli. Further research discovered similarities between the open reading frames of HIV-encoded reverse transcriptase and msDNA, and was later tested for reverse transcriptase activity and RNase H activity. It was found that both the RNase H domain and the enzyme reverse transcriptase were required in order for msDNA to replicate.
The discovery of msDNA has led to questions regarding where reverse transcriptase originated, as genes encoding for it have been found in prokaryotes, eukaryotes, and even viruses. After a DNA fragment coding for the production of msDNA in E. coli was discovered, it was suggested that bacteriophages may have been responsible for the introduction of the RT gene into E. coli. These discoveries suggest that reverse transcriptase played a role in the evolution of viruses from bacteria, with one hypothesis stating that with the help of reverse transcriptase, viruses may have arisen as a breakaway msDNA gene which acquired a protein coat.
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