Specialized transduction of D-serine deaminase genes: formation of lysogens that yield high lambda-d dsd/lambda ratios and formation of a dimeric lambda-d dsd.

We have obtained two classes of double lysogens that on induction yield higher titers of lambda-d dsd transducing phage than of helper phage. One class was obtained by lysogenization of strain EM6116 (dsddelta attlambdadelta HfrC) with lambda-dsd type 2 (dsdC+ dsdO+ dsdA+, head-tail substitution). In the absence of either a normal attlambda or the homology of a chromosomal dsd region, the transducing phage integrated at other sites, at least one of which, in strain EM6177, is near the origin of HfrC. On induction, strain EM6177 yields a phage burst of 20 to 50 with a lambdadsd:lambda ratio of 10(4):1. The asnychronously high yield of lambda dsd is attributed to an efficiency of excision greater than that of lambda. The other class was obtained by lysogenization of strain EM1407 (dsdA attlambda+) with lambda-dsd type 2 (dsdO6 dsdA, partial deletion of dsdC). The DNA of mature lambda-dsd type 2 is a complete dimer. It lacks nearly all the phage late genes and b2 and carries about five bacterial genes. It could not be packaged as a monomer but is just within the packaging size limit as a dimer. Models for the derivation of these lambda dsd phages and the high-yielding lysogens are presented.