Bacteriophage T4 mutants hyperproducing gene 17
protein (
Hp17) have been isolated at high frequency by growing gene 17
amber mutants on ochre suppressor strains of Escherichia coli. Most mutants showed the co-hyperproduction of gene 18
protein, although one anomalous mutant hyperproduced a 60,000 Mr partial
polypeptide of gene 18. Hybridization of T4 late RNAs to cloned plasmid
DNA containing genes 17, 18 or control T4 genes revealed that approximately five times more gene 17
mRNA and two to three times more gene 18
mRNA were synthesized in the
Hp17 mutant
infections.
DNA-
DNA hybridizations showed that
Hp17 mutant
DNA contained two to three times more copies of genes 17 and 18 than wild-type
DNA. Southern blot analysis suggested that
Hp17 mutants carry a direct tandem repeat of the gene 17-18 region, with variable copy number from one to at least six copies. Hyperproduction of gene 17 and 18
proteins appears therefore to result from gene amplification specific to the gene 17-18 region. In contrast to gene duplications reported in lambda and T4 phage, and numerous cases of gene amplification in bacteria, a similar or identical novel junctional fragment created by the amplification event was observed among 28 independent T4
Hp17 isolates; therefore, the mechanism giving gise to amplified sequences may involve specific sequences in this region of the T4 genome.