Role of orf73 in the development of lambdoid bacteriophages during infection of the Escherichia coli host

Shiga toxin-producing Escherichia coil (STEC) is a group of pathogenic strains responsible for human infections that result in bloody diarrhea and hemorrhagic colitis, often with severe complications. The main virulence factors of STEC are Shiga toxins encoded by the six genes located in genomes of Shiga toxin-converting bacteriophages (Six phages). These bacterial viruses are clustered in the lambdoid bacteriophage family represented by phage lambda. Here, we report that expression of orf73 from the exo-xis region of the phage genome promotes the lysogenic pathway of development of lambda and Phi 24(B) phages. We demonstrated that the mutant phages with deletions of orf73 revealed higher burst size during the lytic cycle. Moreover, survival rates of E. coil infected with mutant bacteriophages were lower relative to wildtype viruses. Additionally, orf73 deletion negatively influenced the lysogenization process of E. coli host cells. We conclude that orf73 plays an important biological role in the development of lambdoid viruses, and probably it is involved in the network of molecular mechanisms of the lysis-vs.-lysogenization decision.