Arbitrium-coding phages use peptides to communicate and coordinate the decision between lysis and lysogeny. However, the process by which these phages establish lysogeny remains incompletely understood. Here, focusing on the SPbeta phage family's model phages phi3T and SPβ, we report that a six-gene operon called the "SPbeta phages repressor operon" (Sro) expresses not one but two master repressors, SroE and SroF, the latter of which folds like classical phage integrases. To promote lysogeny, these repressors bind to multiple places in the phage genome. SroD is an additional auxiliary repressor that, along with SroEF, forms the repression module necessary for lysogeny establishment and maintenance. Additionally, the proteins SroABC within the operon are proposed to constitute the transducer module, connecting the arbitrium communication system to the activity of the repression module. Overall, this research sheds light on the intricate and specialised repression system employed by arbitrium SPβ-like phages in making lysis-lysogeny decisions.
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