Characterization of a unique repression system present in arbitrium phages of the SPbeta family.

Arbitrium-coding phages use peptides to communicate and coordinate the decision between lysis and lysogeny. However, the mechanism by which these phages establish lysogeny remains unknown. Here, focusing on the SPbeta phage family's model phages phi3T and SPß, 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 a classical phage integrase. To promote lysogeny, these repressors bind to multiple sites in the phage genome. SroD serves as an auxiliary repressor that, 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 specialized repression system employed by arbitrium SPß-like phages in making lysis-lysogeny decisions.

Integration of a foreign gene into a native complex does not impair fitness in an experimental model of lateral gene transfer.

Lateral gene transfer (LGT) is a central force in microbial evolution. The observation that genes encoding subunits of complexes exhibit relatively compatible phylogenies, suggesting vertical descent, can be explained by different evolutionary scenarios. On the one hand, the failure of a new gene product to correctly interact with preexisting protein subunits can make its acquisition neutral-a theory termed the "complexity hypothesis." On the other hand, foreign subunit-encoding genes may reduce the fitness of the new host by disrupting the stoichiometric balance between complex subunits, resulting in purifying selection against gene retention. We previously showed in a model LGT system that overexpression of an orthologous subunit was neutral due to lack of interaction with host subunits. Here, we examine a case where the foreign protein is more similar to its native orthologs, by expressing the RNA polymerase ß subunit (RpoB) of Bacillus subtilis in Escherichia coli. The foreign subunit is shown by coimmunoprecipitation to interact with the host subunits, and to form novel, nonspecific interactions. Nevertheless, the host did not incur any fitness disadvantage, as measured by its growth. We conclude that LGT of complex subunits may be neutral even when the transferred subunit can integrate into the host complex and that this neutrality can be a fertile ground for selective forces once the environment changes.