DNA Synthesis Associated with Recombination II. Recombination between Repressed Chromosomes

Abstract

In the companion paper it was demonstrated, under conditions of depressed duplication, that recombination in most regions of the λ chromosome is associated with extensive DNA synthesis. Evidence, not fully conclusive, was presented that the rec system shows such behavior. In this paper, we present similar studies employing a different method for depressing DNA duplication; our density-labeled λ crosses were carried out in λ lysogens in the presence of a heteroimmune “helper” phage, which provides the recombination systems and the wherewithal for maturation of the repressed λ chromosomes (see Russo, Stahl, and Stahl, 1970). The results are like those of the companion paper. Evidence is presented that recombination mediated by the red system is associated with DNA synthesis.

MATERIALS AND METHODS
Execution of crosses
Crosses between density-labeled phages in non-labeled bacteria were conducted as follows. An appropriately lysogenized derivative (see Results) of the su⁺ strain QR47 or of its nearly isogenic rec⁻ derivative, QR48, was grown to 1.5 × 10⁸ per ml. A volume of cell suspension was added to an equal volume of phage mixture composed of each of the two labeled parents at 1.5 × 10⁹/ml and the appropriate 434hy “helper” phage (see Results) at 5 × 10⁸/ml. Nonlabeled glucose was added in excess to dilute the heavy atoms added to the mixture along with the (unpurified) phage stocks. Lysates were fractionated by centrifugation as in paper I in most cases. In a few experiments, the ratio of lysate to D₂O was lower.

Determination of phage titers in...