Central dogma, redefined

Science Adv. 7, eabf1771 (2021)

DNA polymerase θ (Polθ) promotes microhomology-mediated end-joining for DNA repair by extending partially annealed single-stranded DNA at double-strand break (DSB) sites. Chandramouly, Zhao and McDevitt et al. found that human Polθ possesses robust reverse transcriptase (RT) activity in vitro, comparable to retrovirus RTs. Further characterization revealed that Polθ incorporated dNTPs at a higher rate and with higher fidelity using RNA templates versus DNA templates. The crystal structure of Polθ in complex with hybrid DNA/RNA revealed a conformational change in the thumb subdomain that enables accommodation of the thicker A-form DNA/RNA hybrid. Binding of the DNA/RNA hybrid was stabilized by hydrogen bonds between Polθ and ribose groups along the RNA template. Using a split GFP reporter, the authors confirmed that Polθ was capable of promoting RNA-templated DNA repair in cells. Moreover, Polθ could use a ribonucleotide-containing donor template to repair CRISPR–Cas9-mediated DSBs. Although the physiological function of Polθ RT activity needs further exploration, this study reveals that RT activity exists in human cells and suggests the diverse ways that nature maintains the stability of genetic information.