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Reinitiation mutants of geneBof bacteriophage S13 that mimic geneAmutants in blocking replicative form DNA synthesis

Authors
Journal
Journal of Molecular Biology
0022-2836
Publisher
Elsevier
Publication Date
Volume
103
Issue
3
Identifiers
DOI: 10.1016/0022-2836(76)90218-7
Disciplines
  • Biology

Abstract

Abstract Several mutations in gene B of phage S13 appear to shorten the B protein by elimination of an N-terminal fragment, without destroying the B protein function. The shortened B protein resulting from each of these mutations can block the unique DNA-nicking properties of the S13 gene A protein. Because of the block in gene A function, normal gene B protein may have a function in phage DNA synthesis in addition to its known role in catalyzing capsid assembly. From gel electrophoresis the mutant B protein is estimated to be shorter than the normal S13 B protein by 1720 ± 70 daltons and is therefore believed to be an internal reinitiation fragment. The reinitiated fragments are functional and are made in about twice the amount of the normal B protein. The phage mutants which yield the reinitiation fragments are double mutants, each phage containing the same gene B nonsense mutation and each appearing to contain a different compensating gene B mutation. Various data support the assumption that the compensating mutations are frame-shifts, including the fact that suppression does not restore the normal-sized B protein. The reinitiation is assumed to occur at a pre-existing out-of-phase initiator codon, near the nonsense triplet; the correct reading frame would then be restored by each of the several different compensating mutations. The position of the normal S13 B protein in the gel electrophoresis pattern has been located both by elimination and shifting of the B peak, using appropriate amber mutants. The molecular weight of the S13 B protein is about 17,200, and is 2100 daltons less than the B protein of phage φX174; the S13 B protein can nevertheless substitute for the φX 174 B protein. Thus substantial portions of the B protein can be deleted without destroying its function.

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