Genetics of the left arm of the chromosome of bacteriophage lambda.

HE chromosome of temperate bacteriophage lambda consists of three functionally distinct regions of approximately equal size. The right arm contains the genes for regulation (EISEN et al. 1966), DNA synthesis (JOYNER et al. 1966) and cell lysis (HARRIS (et al. 1967). The central portion of the h DNA molecule is concerned with the processes of prophage integration (KELLENBERGER, ZICHICHI and WEIGLE 1961 ; ZISSLER 1967) and vegetative recombination (FRANKLIN 1967). Neither of these latter functions is essential for vegetative growth and the DNA in this region can be deleted with no loss of viability (FRANKLIN 1967; HUSKEY 1968). The left arm of the lambda chromosome is comprised of genes controlling DNA maturation (DOVE 1966; SALZMAN and WEISSBACH 1967) and genes which code for the structural components of the lambda phage particle ( WEIGLE 1966). Although many studies have been devoted to the right and central portions of the lambda chromosome, the little that we know about the left arm is mostly of a qualitative nature. CAMPBELL (1961 ) isolated suppressor-sensitive (sus) nonsense mutants of h and defined 13 complementation classes in the left arm on the basis of complementation spot tests. He also ordered these 13 classes with respect to one another by deletion mapping with hdg transducing phage. The present study was prompted by this lack of quantitative genetic information concerning the left arm of the lambda chromosome. Many of the sus mutants of CAMPBELL cannot be used to analyze the left arm in a quantitative manner. This is due to the fact that CAMPBELL’S mutants give small burst sizes under permissive conditions, suggesting that the mutant phenotype is not fully suppressed in the permissive host. To obtain reproducible recombination values in order to establish the size and spatial arrangement of genes in the left arm of the h genome, the burst size of sus mutants under permissive growth conditions should approach that of lambda wild type. Therefore, new conditions for performing crosses have been devised and new sus mutants have been isolated which appear to be fully suppressed in the permissive host. The complementation between genes in the left arm has been studied by measuring the total phage yield of non-permissive cells which have been infected with two different sus mutants. Evidence is presented that the left arm of the lambda