Modulation of Lactobacillus casei bacteriophage A2 lytic/lysogenic cycles by binding of Gp25 to the early lytic mRNA

The genetic switch of Lactobacillus casei bacteriophage A2 is regulated by the CI protein, which represses the early lytic promoter P and Cro that abolishes expression from the lysogenic promoter P. Lysogens contain equivalent cI and cro-gp25 mRNA concentrations, i.e., CI only partially represses P, predicting a lytic cycle dominance. However, A2 generates stable lysogens. This may be due to Gp25 binding to the cro-gp25 mRNA between the ribosomal binding site and the cro start codon, which abolishes its translation. Upon lytic cycle induction, CI is partially degraded, cro-gp25 mRNA levels increase, and Cro accumulates, launching viral progeny production. The concomitant concentration increase of Gp25 restricts cro mRNA translation, which, together with the low but detectable levels of CI late during the lytic cycle, promotes reentry of part of the cell population into the lysogenic cycle, thus explaining the low proportion of L.casei lysogens that become lysed (∼1%). A2 shares its genetic switch structure with many other Firmicutes phages. The data presented may constitute a model of how these phages make the decision for lysis versus lysogeny. A2 shares its genetic switch organization with many temperate Firmicutes phages, suggesting a common lysis/lysogeny commitment. The lytic early promoter of A2 is so potent that it cannot be completely repressed by CI. This should lead to permanent lytic cycle activation. However, A2 efficiently forms lysogens because Gp25 binds the untranslated region of the cro mRNA and aborts its translation and that of the replication genes. The ecological implications of this regulatory network are discussed.This work was supported by the Ministerio de Economía y Competitividad (BFU2007-65781 and AGL2010-15097 to J.E.S.; BFU2012-39879-C02-01 to J.C.A.), the Fondo Europeo de Desarrollo Regional Plan (AGL2010-15097) and the Programa de Ciencia, Tecnología e Innovacion of the Principado de Asturias and FEDER Program GRUPIN14-139) to J.E.S. We thank J.R. Penadés (Univ. of Glasgow) and S. Ayora (CNB-CSIC) for critical reading of the article. B.C. was a holder of a Juan de la Cierva contract from the Ministerio de Economía y Competitividad.Peer Reviewe