A team of researchers from Thomas Jefferson University, Philadelphia, the University of Southern California, the Beckman Research Institute of the City of Hope, and the New York University School of Medicine has provided the first evidence that RNA sequences can be written back into DNA, a feat more common in viruses than eukaryotic cells.
“This work opens the door to many other studies that will help us understand the significance of having a mechanism for converting RNA messages into DNA in our own cells,” said senior author Dr. Richard Pomerantz, a researcher in the Department of Biochemistry and Molecular Biology at Thomas Jefferson University.
“The reality that a human polymerase can do this with high efficiency, raises many question.”
“For example, this finding suggests that RNA messages can be used as templates for repairing or re-writing genomic DNA.”
In their study, Dr. Pomerantz and colleagues focused on a very unusual polymerase called polymerase theta (Polθ).
Of the 14 DNA polymerases in mammalian cells, only three do the bulk of the work of duplicating the entire genome to prepare for cell division.
The remaining 11 are mostly involved in detecting and making repairs when there’s a break or error in the DNA strands.
Polθ repairs DNA, but is very error-prone and makes many errors or mutations.
The scientists noticed that some of Polθ’s qualities were ones it shared with another cellular machine, albeit one more common in viruses — the reverse transcriptase.
Like Polθ, HIV reverse transcriptase acts as a DNA polymerase, but can also bind RNA and read RNA back into a DNA strand.
In a series of experiments, the authors tested Polθ against the reverse transcriptase from HIV, which is one of the best studied of its kind.
They showed that Polθ was capable of converting RNA messages into DNA, which it did as well as HIV reverse transcriptase, and that it actually did a better job than when duplicating DNA to DNA.
Polθ was more efficient and introduced fewer errors when using an RNA template to write new DNA messages, than when duplicating DNA into DNA, suggesting that this function could be its primary purpose in the cell.
Using X-ray crystallography, the team found that this molecule was able to change shape in order to accommodate the more bulky RNA molecule — a feat unique among polymerases.
“Our research suggests that Polθ’s main function is to act as a reverse transcriptase,” Dr. Pomerantz said.
“In healthy cells, the purpose of this molecule may be toward RNA-mediated DNA repair.”
“In unhealthy cells, such as cancer cells, Polθ is highly expressed and promotes cancer cell growth and drug resistance.”
“It will be exciting to further understand how Polθ’s activity on RNA contributes to DNA repair and cancer-cell proliferation.”
The study was published in the journal Science Advances.
Gurushankar Chandramouly et al. 2021. Polθ reverse transcribes RNA and promotes RNA-templated DNA repair. Science Advances 7 (24): eabf1771; doi: 10.1126/sciadv.abf1771