258 Structure of Viruses /I4 : 5 



configurations are used to determine whether two mutations are in the 

 same cistron. In the trans configuration, the two mutations are on 

 different members of a pair of homologous chromosomes. If they lead 

 to no normal (wild-type) offspring, the mutations are called noncomple- 

 mentary. Two noncomplementary mutations are said to be in the same 

 cistron provided that one normal chromosome can lead to normal 

 offspring. This is checked in the cis configuration, with both mutations 

 on the same chromosome. 



In bacteriophage genetics, each phage particle is considered to be 

 homologous to a chromosome. The wild-type phage is the homolog of 

 the normal chromosome. The cis configuration of mixed rll-mutants 

 and wild-type phage always leads to (normal) wild-type plaques (off- 

 spring). Thus, the trans configuration, a mixture of two rll-mutants, 

 plated on E. coli K, can be used to determine whether the two mutants 

 are in the same cistron. Studies with more than 200 different rll- 

 mutants of T4 phages have shown that the "rll-gene" consists of two 

 cistrons. When phage strains with mutations in different cistrons are 

 mixed, grown on E. coli B, and plated on E. coli K, many wild-type 

 plaques are found. By way of contrast, wild-type plaques are rarely 

 found when mutants in the same cistron are mixed. Thus, the study of 

 T4 genetics of the rll-mutants shows the existence of two cistrons which, 

 together, may be considered to make up the rll-gene. 



Just as the gene is divided into cistrons by a study of recombination 

 probabilities, it is likewise possible to subdivide the cistron into smaller 

 units called recons on the basis of similar recombination data. Although 

 the probabilities for recombinations within the same cistron are small, 

 they are not zero. A large number of experiments have indicated that 

 the linear separation of two mutations is directly proportional to the 

 probability of recombination. The advantage of the rll-mutants of 

 T4 phage is that hundreds of mutants per cistron can be found, and that 

 these mutations, although lethal on E. coli K, can be propagated on 

 E. coli B. In this fashion, it is possible to show that the closest pairs of 

 mutants, which will recombine to give wild-type plaques, are separated 

 by a distance corresponding to a probability of 0.01 per cent. The ones 

 farthest apart within a cistron are separated by a distance corresponding 

 to 4 per cent. Thus, one may say the basic unit, the recon, is 0.01- 

 0.02 per cent long whereas the cistron is 4 per cent long. Maps, such 

 as the one in Figure 6, have been made showing the separation in 

 recons of the various mutations. The recon length may be expressed in 

 terms of physical lengths along the DNA chain if a few assumptions are 

 made. These have led to an estimated length for the recon of about 

 10 A, a length comparable to the separation of the monomers (nucleotide 

 pairs) along the DNA chain (3.4 A). 



