Studies on the bacteriophage MS2: XLI. Nature of the azure mutation |
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Authors: | D. Iserentant M. Van Montagu W. Fiers |
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Affiliation: | 1. Laboratory of Molecular Biology State University of Ghent Ledeganckstraat, 35 B-9000 Ghent, Belgium;2. Laboratory of Histology State University of Ghent Ledeganckstraat, 35 B-9000 Ghent, Belgium |
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Abstract: | Azure (or reverse amber) mutants grow normally on wild-type Escherichia coli but not on host strains harbouring a strong UAG suppressor mutation. Three different bacteriophage MS2 azure mutants obtained by treatment with nitrous acid have been characterized at the nucleotide sequence level. The 3′-end fragment of the 32P-labelled mutant genomes was isolated by DNA:RNA hybridization and treatment with nuclease S1, and was analyzed by mini-fingerprinting of the RNA. It is known that the wild-type MS2 polymerase gene ends with a UAG codon, followed seven triplets further by an in-phase UAA triplet. All three azure mutants contained an A → G transition in this UAA second stop codon of the polymerase gene, resulting in a second suppressible UAG (amber) codon. Analysis of revertants demonstrated that the azure mutation can be counteracted either by a true site reversion at the second stop or by the creation of a new stop signal for the polymerase gene, either UAA (ochre) or UGA (opal), before or at the first stop, or beyond the second stop. On the basis of these results, a mechanism for the azure mutation is proposed. Silent mutations (one in the coding region, three in the untranslated 3′-terminal sequence) have also been observed in these phage stocks. |
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Keywords: | Author to whom all correspondence should be addressed |
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