GSBS Dissertations and Theses

Approval Date

June 1993

Document Type

Doctoral Dissertation

Department

Graduate School of Biomedical Sciences, Molecular Genetics & Microbiology

Subjects

Bacteriophage P22; Chromosome Mapping; Superinfection; Academic Dissertations; Dissertations, UMMS

Abstract

The superinfection exclusion gene (sieB) of Salmonella phage P22 was mapped using phage deletion mutants. The DNA sequence in the region was re-examined in order to find an open reading frame consistent with the deletion mapping. Several discrepancies from the previously published sequence were discovered. The revised sequence revealed a single open reading frame of 242 codons with six likely translation initiation codons. On the basis of deletion and amber mutant phenotypes the second of these six sites was inferred to be the translation initiation site of the sieB gene. The sieB gene encodes a polypeptide with 192 amino acid residues with a calculated molecular weight of 22,442, which is in reasonable agreement with that estimated from polyacrylamide gels. The transcription start-site of sieB was identified by the use of an RNAase protection assay. The sieB promoter thus identified was inactivated by a two-base substitution in its -10 hexamer. The sieB gene of coliphage λ was also identified. The promoter for λ sieB was identified by homology to that of P22 sieB.

sieB aborts the lytic development of some phages. P22 itself is insensitive to the lethal effect of SieB because it harbours a determinant called esc. It was found that the sieB gene encodes two polypeptides-SieB, which is the exclusion protein, and Esc, which is a truncated version of SieB that inhibits its action. Superinfecting P22 synthesizes an antisense RNA, sas, that inhibits synthesis of SieB but allows continued synthesis of Esc, thus allowing P22 to by-pass SieB-mediated exclusion. This translational switch induced by sas RNA is essential to vegetatively developing P22; a mutation that prevents this switch causes P22 to commit SieB-mediated suicide. It was also found that P22's Esc allows it to circumvent the SieB-mediated exclusion system of bacteriophage λ.

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