33. NUCLEIC ACIDS OF THE BACTERIAL VIRUSES 217 



phages is the genetic determinant of the phage. Hershey's experiments 

 demonstrate that DXA is the major substance to enter the host upon in- 

 fection. Genetic experiments with mixed infection indicate that during the 

 infection a pool of phage genomes is accumulated which undergoes several 

 rounds of recombination before lysis. 134 The physical correlate to ibis 

 genetic pool appears to be the phage DNA, for recombination is observed 

 to take place in the presence or absence of chloramphenicol which inhibits 

 protein synthesis. 106 ' 135 



The most direct approach to the determination of the physical size of a 

 genetic determinant of function in T4 phage has come from the genetic 

 fine structure experiments of Benzer. 136 ' 137 



All of the mutant characters known in a T-even phage can be mapped 

 in a linear order — a genetic map — on which the characters are separated 

 by distances proportional to the recombination frequencies observed when 

 crosses are made. 9 The total length of this map — the sum of the recombina- 

 tion frequencies between adjacent markers — is estimated to be 200-800 

 units and is assumed to correspond to the DXA content, 200,000 nucleotide 

 pairs, of the phage (or to a portion of the DNA content if some of the 

 DXA is assumed to be nongenetic). 



A particular mutant locus — rll — can be recognized by the property 

 that a phage bearing this mutant locus produces unusually large plaques 

 when plated on E. coli B and produces effectively no plaques when plated 

 on E. coli K12 (X). Many mutants, both natural and induced, have been 

 located in the rll region. Since it is possible to select for recombinants 

 from crosses of rll mutants by plating the progeny on K12 (X) — only the 

 wild-type recombinants will plate — it is possible to map this class of mu- 

 tants very precisely, i.e., to very low recombination frequencies. It has been 

 shown that the extremes of the rll region encompass at least eight map 



un i ts 138, 138a 



An independent feature of the rll region is that it can be subdivided 

 into two, approximately equal, loci known as cistrons (A and B) which 

 have distinct functions; when a K12 (X) cell is infected with two rll mu- 

 tant phages one of which is mutant in the A cistron and one in the B cis- 

 tron the infection will be successful and progeny (mostly of the parental 

 types) will be produced. 



134 N. Visconti and M. Delbriick, Genetics 38, 5 (1953). 



135 A. D. Hershey, E. Burgi, and G. Streisinger, Virology 6, 287 (1958). 



136 S. Benzer, Proc. Natl. Acad. Sci. U. S. 41, 344 (1955). 



137 S. Benzer, in "The Chemical Basis of Heredity" (W. D. McElroy and B. Glass, 

 eds.), p. 70. Johns Hopkins, Baltimore, 1957. 



138 R. S. Edgar, Virology 6, 215 (1958). 



n8a Two genetic loci are said to be one "map unit" apart when the frequency of 

 recombinant types among the progeny of a cross, involving the two loci, is 1%. 



