Marker effects in the genetic transduction of tryptophan mutants of E. coli.

Recombination frequencies have been determined in crosses involving 28 mutant strains for 20 of which the site of the alteration is known from studies of amino-acid substitutions in the protein products. Three of these mutants showed especially high frequencies of recombination when crossed to other single mutants or when crossed to a strain carrying two alterations at opposite ends of the trpA gene. There is no obvious molecular explanation of the high recombination of these three mutants. They include one missense mutant, one amber and one ochre. The low-frequency recombination mutants include all these same classes as well as frameshift mutants. There is nothing unique about the intragenic location of the high-recombination mutants; in each case there is at least one low-recombination mutant in the same codon.-Crosses involving mutants which were isolated in an altered wild type have shown that the behavior of a high-recombination mutant does not result from its molecular configuration alone, but from its combination with the homologous wild-type sequence from the other parent.-Several lines of evidence indicate that recombination in this system frequently involves closely-spaced double exchanges (about 40 codons apart).