Multicopy single-stranded DNA

An example of an RNA stem-loop, similar to what is seen in the RNA region of msDNA

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.

References

• Reverse Transcriptase in E.coli: msDNA — Mellon College of Science

Footnotes

1. ^ Inouye S, Herzer PJ, Inouye M (February 1990). "Two independent retrons with highly diverse reverse transcriptases in Myxococcus xanthus". Proc. Natl. Acad. Sci. U.S.A. 87 (3): 942–5. doi:10.1073/pnas.87.3.942. PMC 53385. PMID 1689062. http://www.pnas.org/cgi/pmidlookup?view=long&pmid=1689062. 
2. ^ Lampson BC, Inouye M, Inouye S (2005). "Retrons, msDNA, and the bacterial genome". Cytogenet. Genome Res. 110 (1-4): 491–9. doi:10.1159/000084982. PMID 16093702. http://content.karger.com/produktedb/produkte.asp?typ=fulltext&file=CGR20051101_4491. 
3. ^ Lampson BC, Sun J, Hsu MY, Vallejo-Ramirez J, Inouye S, Inouye M (February 1989). "Reverse transcriptase in a clinical strain of Escherichia coli: production of branched RNA-linked msDNA". Science 243 (4894 Pt 1): 1033–8. doi:10.1126/science.2466332. PMID 2466332. http://www.sciencemag.org/cgi/pmidlookup?view=long&pmid=2466332. 
4. ^ Hsu MY, Inouye M, Inouye S (December 1990). "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. U.S.A. 87 (23): 9454–8. doi:10.1073/pnas.87.23.9454. PMC 55184. PMID 1701261. http://www.pnas.org/cgi/pmidlookup?view=long&pmid=1701261. 
5. ^ Inouye S., Inouye M. (1993). "Bacterial Reverse Transcriptase". In Goff, Stephen; Skalka, Anna M.. Reverse transcriptase. Cold Spring Harbor monograph series. 23. Plainview, N.Y: Cold Spring Harbor Laboratory Press. ISBN 0-87969-382-7.