Mammalian Cells Can Convert RNA Segments Back Into DNA, New Research Reveals | Biology – Sci-News.com

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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.

Ternary structure of Polθ on a DNA/RNA primer-template: (A) Polθ polymerase; (B) DNA/RNA extension by Polθ and PolθΔL; (C) structure of Polθ:DNA/RNA:ddGTP; (D) superposition of Polθ:DNA/RNA (marine) and Polθ:DNA/DNA (orange, 4x0q); the fingers and thumb subdomains undergo reconfiguration; (E) superposition of Polθ:DNA/RNA (marine) and Polθ:DNA/DNA (orange, 4x0q) highlighting a 12-Å shift of K2181 (blue box; thumb) and a 4.4-Å shift of E2246 (gray box; palm); (F) superposition of nucleic acids and ddGTP from Polθ:DNA/RNA:ddGTP and Polθ:DNA/DNA:ddGTP structures; (G) top: electron density of ddGTP and 3′ primer terminus in Polθ:DNA/RNA structure; bottom: zoomed-in image of the superposition of active sites, illustrating a different conformation of ddGTP in the Polθ:DNA/RNA (blue) and Polθ:DNA/DNA (salmon) complexes; (H) interactions between ribose 2’-hydroxyl groups of the RNA template and residues in the Polθ:DNA/RNA structure; red dashed lines, hydrogen bonds; (I) DNA/RNA used for cocrystallization with Polθ and ddGTP (top); strong electron density is present for four base pairs [nucleotides located at positions 2 to 5 (underlined) of the DNA/RNA] and two base pairs resulting from an incorporated ddGMP (2’,3’ dideoxyguanosine monophosphate) (green; position 1) and a bound unincorporated ddGTP (red; position 0) in the active site (top); interactions between Polθ and nucleic acids in Polθ:DNA/RNA:ddGTP (bottom); interactions between residues and phosphate backbone, sugar oxygen, or nucleobase are shown in blue, yellow, and green, respectively; hydrogen bonds between Polθ and ribose 2’-hydroxyl groups are indicated (boxed residues); (J) interactions between Polθ and nucleic acids in Polθ:DNA/DNA:ddGTP (4x0q); color scheme identical to (I). Image credit: Chandramouly et al., doi: 10.1126/sciadv.abf1771.

“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.

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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