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Lambdoid bacteriophages form a group of viruses that shares a common schema of genome organization and lifecycle. Some of them can play crucial roles in creating the pathogenic profiles of  Escherichia coli  strains. For example, Shiga toxin-producing  E. coli  (STEC) acquired  stx  genes, encoding Shiga toxins, via lambdoid prophages (Stx phages). The results obtained so far present the evidence for the relation between the  exo-xis  region of the phage genome and lambdoid phage development, however molecular mechanisms of activities of the  exo-xis  genes' products are still unknown. In view of this, we decided to determine the influence of the uncharacterized open reading frame  orf63  of the  exo-xis  region on lambdoid phages development using recombinant prophages, λ and Stx phage Φ24 B.  We have demonstrated that  orf63  codes for a folded protein, thus, it is a functional gene. NMR spectroscopy and analytical gel filtration were used to extend this observation further. From backbone chemical shifts, Orf63 is oligomeric in solution, likely a trimer and consistent with its small size (63 aa.), is comprised of two helices, likely intertwined to form the oligomer. We observed that the deletion of phage  orf63  does not impair the intracellular lambdoid phage lytic development, however delays the time and decreases the efficiency of prophage induction and in consequence results in increased survival of  E. coli  during phage lytic development. Additionally, the deletion of phage  orf63  negatively influences expression of the major phage genes and open reading frames from the  exo-xis  region during prophage induction with hydrogen peroxide. We conclude, that lambdoid phage  orf63  may have specific functions in the regulation of lambdoid phages development, especially at the stage of the lysis vs. lysogenization decision. Besides,  orf63  probably participates in the regulation of the level of expression of essential phage genes and open reading frames from the  exo-xis  region during prophage induction.

Bad Phages in Good Bacteria: Role of the Mysterious orf63 of λ and Shiga Toxin-Converting Φ24B Bacteriophages