354 GENETICS: S. BENZER Proc. N. A. S. 



would seem feasible, with this system, to extend genetic studies even to the level 

 of the latter functional elements. 



Summary. — It has been discovered that the mutations in the rll region of phage 

 T4 have a characteristic in common which sets them apart from the mutations 

 in all other parts of the map. This characteristic is a host-range reduction, namely, 

 a failure to produce plaques on a host (K) lysogenic for phage X. The mutant phage 

 particles adsorb to and kill K, but normal lysis and phage release do not occur. 



All mutants with this property are located within a sharply defined portion of 

 the phage linkage map. Within that region, however, their locations are widely 

 scattered. An unambiguous seriation of the mutants, with roughly additive dis- 

 tances, can be accomplished, except for certain anomalous cases. 



The simultaneous presence of a wild-type phage particle in K enables the multi- 

 plication of rll mutants to proceed, apparently by supplying a function in which 

 the mutant is deficient. A heterozygous diploid in the trans configuration is 

 simulated by a mixed infection of K with two mutant types. The application of 

 the phenotype test to pairs of rll mutants leads to the division of the region into 

 two functionally separable segments. 



Spontaneous reversion to wild-type had been observed for most of these mutants. 

 It remains to be seen whether these are genuine reversions. Each mutant reverts 

 at a characteristic rate, but the rates for different mutants differ enormously. 

 Partial reversions to intermediate types are also observed. 



The mutants differ greatly in degree of residual ability to grow on K. There is 

 no evident correlation between map position, reversion rate, and degree of residual 

 activity of the various mutants. 



The selective feature of K for wild-type recombinants offers the possibility of 

 extending the recombination studies to an analysis of the fine details of the region. 



Preliminary studies of this type indicate that the units of recombination are not 

 larger than the order of one dozen nucleotide pairs and that mutations may involve 

 various lengths of "chromosome." 



I am much indebted to A. D. Hershey and A. H. Doermann for stocks of their 

 genetically mapped mutants, to Sydney Brenner and David Krieg for stimulating 

 discussion, and to Max Delbriick for his invaluable moderating influence. 



* Supported by a grant-in-aid from the American Cancer Society upon recommendation of the 

 Committee on Growth of the National Research Council. 



1 E. B. Lewis, Cold Spring Harbor Symposia Quant. Biol., 16, 159-174, 1951. 



2 G. Pontecorvo, Advances in Enzymol., 13, 121-149, 1952. 



3 A. D. Hershey and M. Chase, /. Gen. Physiol, 36, 39-56, 1952. 



* A. D. Hershey, J. Di.xon, and M. Chase, /. Gen. Physiol, 36, 777-789, 1953. 

 ^ E. K. Volkin, personal communication. 



6 J. D. Watson and F. H. C. Crick, Cold Spring Harbor Symposia Quant. Biol, 18, 123-131, 1953. 



7 A. H. Doermann and M. B. Hill, Genetics, 38, 79-90, 1953. 



* G. Streisinger and V. Bruce, personal communication. 

 9 A. D. Hershey, Genetics, 31, 620-640, 1946. 



10 A. H. Doermann, /. Bacterial, 55, 257-276, 1948. 



II A. D. Hershey and R. Rotman, Genetics, 34, 44-71, 1949. 



12 E. M. Lederberg and J. Lederberg, Genetics, 38, 51-64, 1953. 

 " D. Krieg, personal communication. 

 1^ N. Visconti and M. Delbruck, Genetics, 38, 5-33, 1953. 



1* A. D. Hershey and M. Chase, Cold Spring Harbor Syynposia Quant. Biol, 16, 471-479, 1951; 

 C. Levinthal, Genetics, 39, 169-184, 1954. 



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