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Abstract
SUMMARY: When a strain of Escherichia coli lysogenic for λ and deleted for the gal operon was infected with λdg two types of gal+ transductants arose, an expected class with enzyme levels similar to wild-type, and a exceptional class with elevated levels of enzyme and altered staining reactions on indicator plates. Transductants of the exceptional class appear to carry multiple copies of a homoimmune λdg prophage. When exceptional transductants were prepared carrying a ts kinase mutation in their λdg and then selected for ability to grow at high temperatures, two types of λdg could subsequently be isolated from the temperature-resistant revertants, one carrying a temperature-resistant and the other a temperature-sensitive kinase gene. It has been possible to infer the existence of polytransductants containing more than two transducing prophage by their segregation patterns. The rate at which such cells produce phage, after u.v. induction, in a medium with galactose as the sole carbon source, is accounted for by the polytransductant structure.
Polytransductant cells arose following infection by λdg at very low multiplicities. Identification of prophage types after mixed infection with genetically marked λdg's showed that each polytransductant carried only one type of transducing phage. When the superinfecting transducing phage was unable to make active λ repressor, as is the case for λpg8 -C1857 at high temperature, the frequency of polytransductant formation was enhanced. Inhibition of protein synthesis with chloramphenicol also enhanced polytransductant formation. These experiments suggest that, at least in the case of λdg, limited phage replication does take place in immune hosts, and is followed by multiple integrations into the bacterial chromosome.
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