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Multicopy single-stranded DNA

Multicopy single-stranded DNA (msDNA) is a type of extrachromosomal satellite DNA that 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[1]. Its function remains unknown even though many copies are present within cells[2].

Before the discovery of msDNA in Myxococcus 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; as a result, 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 are required in order for msDNA to replicate[3].

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[4], it was suggested that bacteriophages might have been responsible for the introduction of the RT gene into E. coli[5]. 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 that acquired a protein coat.


* Reverse Transcriptase in E.coli: msDNA - Mellon College of Science


1. ^ Inouye, et al. Two independent retrons with highly diverse reverse transcriptases in Myxococcus xanthus. Proc Natl Acad Sci USA, 87(3), pg 942–945 (1990)
2. ^ Lampson, et al. Retrons, msDNA, and the bacterial genome. Cytogenet Genome Res., 110(1-4), pg 491-9 (2005)
3. ^ Lampson, et al. Reverse Transcriptase in a Clinical Strain of Escherichia coli: Production of a Branched RNA-linked msDNA. Science, 243, pg 1037-1038 (1989)
4. ^ Hyu, et al. Retron for the 67-base multicopy single-stranded DNA from Escherichia coli: a potential transposable element encoding both reverse transcriptase and Dam methylase functions. Proc Natl Acad Sci USA, 87(23), pg 9454–9458 (1990)
5. ^ Inouye, et al. Bacterial Reverse Transcriptases REVERSE TRANSCRIPTASE. Cold Spring Harbor Press. Plainview, NY. (1993)


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