Retrons are bacterial genomic elements encoding a reverse transcriptase and a non-coding RNA (ncRNA). Despite their discovery more than three decades ago, the function of retrons remains unknown. Millman et al. found that many retrons are in close proximity in the genome to known phage-defence systems, suggesting that they are also involved in defence against phage infection. Retrons form a cassette that includes a reverse transcriptase, ncRNA and one or two genes encoding proteins that might function as effectors. Introduction of retrons and their effectors into an Escherichia coli strain lacking retrons provided defence against infection by various phages, by growth arrest or cell death. The authors showed that the defence mechanism of one retron involved sensing phage-mediated inactivation of the phage DNA-degrading RecBCD complex, which activated retron effectors and led to abortive infection.
Millman, A. et al. Bacterial retrons function in anti-phage defense. Cell https://doi.org/10.1016/j.cell.2020.09.065 (2020)