S-12. Genomics and proteonics of bacteriophage MB78

Bacteriophage MB78 isolated in our laboratory is one of the virulent phages of Salmonella enterica serovar typhimurium. It is morphologically, serologically and even physiologically quite different from P22 and related phages. It has 42 kb long, terminally redundant and circularly permuted DNA. It is a dominant phage and does not allow other phages like P22 and 9NA to grow in its presence. It cannot grow in rifampicin resistant mutant of the host (rif39); although the temperate phage P22 can. However, P22 can help MB78 to grow in rif39. A genomic library of the phage was made in M13mp11 and the detailed physical map of 42 kb phage genome has been constructed. More than 80% of the genome has been sequenced. To identify different phage genes and study regulation of their expression, the genomic fragments have been cloned, expressed and studied extensively. Of the promoter-containing fragments the two containing strong promoters have been studied in detail. The phage MB78 uses various methods for regulation of its gene expression. Two late proteins are expressed from the same gene by ribosomal frame shifting. Presence of multiple copy of this gene (through plasmid) in a permissive host interferes with phage morphogenesis. Messages are mostly monocistronic but some proteins are expressed from polycistronic message without any intercistronic gap where stop and start codons overlap and rare initiation codons are used. One of the two proteins expressed from a 0.9 kb Sal I-Hind III fragment has 57% similarity with a structural protein of mycobacteriophage. Usually structural protein genes of bacteriophages are clustered. In MB78, however, location of the structural protein genes are scattered over the genome. A minor structural gene involved in phage morphogenesis could be identified and studied in detail. The presence of restriction-modification system in the phage makes it unique.