Effects of ultraviolet light on transducing phage P22

Abstract

The effects of ultraviolet irradiation on suspensions of P22 particles are examined. Evidence is presented which indicates that a single ultraviolet hit on an abortively transducing element enables it to form a complete transduction, i.e., to undergo recombination with the chromosome of recipient bacteria. Thus, abortive transductions are rapidly inactivated while the number of complete transductions increases. During the conversion of abortive transductions to complete transductions, jointly transduced markers become less tightly linked. The rate of linkage disruption is proportional to the genetic map distance between linked markers and allows an estimate to be made for the physical size of the histidine ( his) region of the Salmonella chromosome. The rates of inactivation of complete transductions are proportional to the sizes of the genetic defects in the recipients, allowing a second estimate of the size of the his region to be made. The eight-gene his region comprises an amount of DNA equivalent to approximately 8 × 10 6 molecular weight, i.e., one-fifth that contained in a normal P22 phage particle. The coding ratio for the his region is about three nucleotide pairs per amino acid in the eight proteins whose structure it controls.