BENZER 



273 



cleotide pairs. We wish to translate 

 linkage distances, as derived from gen- 

 etic recombination experiments, into 

 molecular units. This cannot be done 

 very precisely at present. It is not 

 known whether all the DNA in a 

 phage particle is indispensable genetic 

 material. Nor is it known whether a 

 phage "chromosome" (i.e., the physical 

 counterpart of a linkage group iden- 

 tified bv" genetic means) is composed 

 of a single (duplex) DNA fiber or 

 whether genetic recombination is 

 equally probable in all chromosomal 

 regions. For the purpose of a rough 

 calculation, however, these notions 

 will be assumed to be true. Thus we 

 place the total linkage map of T4 in 

 correspondence with 2 X 10'' nucleo- 

 tide pairs of DNA. The total known 

 length of the three linkage groups '^ in 

 phage T4 amounts to some 100 units 

 (one unit = 1 per cent recombination 

 in a standard cross). In addition, there 

 is evidence ^ for roughly another 100 

 units of length connecting two of the 

 groups. Therefore, if we assume 200 

 recombination units to correspond to 

 2 X 10^ nucleotide pairs, the recom- 

 bination per nucleotide pair is 10"^ per 

 cent. That is to say, given two phage 

 mutants whose mutations are localized 

 in their chromosomes at sites only one 

 nucleotide pair apart, a cross between 

 these mutants should give rise to a 

 progeny population in which one par- 

 ticle in 10^ results from recombination 

 between the mutations (provided, of 

 course, that recombination is possible 

 between adjacent nucleotide pairs). 

 This computation is an exceedingly 

 rough one and is only intended to in- 

 dicate the order of magnitude of the 

 scale factor. Some preliminary results 

 are here presented of a program de- 



1 Doermann, A. H., and HUl, M. B., Ge- 

 Jietics 38:79-90, 1953. 



s Streisinger, G., and Bruce, V., personal 

 communication. 



signed to extend genetic studies to the 

 molecular (nucleotide) level. 



r Aiutcmts.— The wild-type phages 

 T2, T4, and T6 produce small plaques 

 with rough edges when plated on 

 strain B of Escherichia coli. From sec- 

 tors of clearing in these plaques, mu- 

 tants can be readily isolated which 

 produce large, sharp-edged plaques 

 (Hershey "). These mutants have been 

 designated "r" for rapid lysis; they dif- 

 fer from the wild type by a failure to 

 cause "lysis inhibition" on strain B 

 (Doermann ^^). The wild type has a 

 selective advantage over r mutants 

 when the two types grow together on 

 B. The genetics of r mutants was 

 studied by Hershey and Rotman,^^ 

 who found three regions in the link- 

 age map of T2 in which various muta- 

 tions causing the r phenotype were 

 located, including one large "cluster" 

 of mutants which were shown to be 

 genetically distinct from one another. 

 The genetic study of T4 by Doermann 

 and Hill '' showed r regions corre- 

 sponding to two of those in T2. T6 

 also has at least two such r regions. 



The rll Group.— For all three 

 phases, T2, T4, and T6, the r mutants 

 can be separated into groups on the 

 basis of their behavior on strains other 

 than B. This paper will be concerned 

 only with one group, which will be 

 called the "rll group." A^Iutants of the 

 rll group are distinguished from those 

 of other groups, and from wild type, 

 by a failure to produce plaques on cer- 

 tain lysogenic strains ^- of E. coli 

 which carry phage >-. As shown in 

 Table 1, a mutant of the rll group 



9 Hershey, A. D., Genetics 31:620-640, 

 1946. 



10 Doermann, A. H., /. Bacterial. 55:257- 

 276, 1948. 



11 Hershey, A. D., and Rotman, R., Ge- 

 netics 34:44^71, 1949. 



12 Lederberg, E. M., and Lederberg, J., 

 Genetics 38:51-64, 1953. 



