Vol. 41, 1955 GENETICS: S. BENZER 353 



Discussion. — ^The set of rll mutants defines a bounded region of a linkage group 

 in which mutations may occur at various locations, all the mutations leading to 

 qualitatively similar phenotypic effects. The rll region would seem, therefore, to 

 be functionally connected, so that mutations arising anywhere within the region 

 affect the same phenotype. This effect is expressed, in case strain B is the host, 

 by failure to produce lysis inhibition; in case S is the host, by no consequence; and 

 in case K is the host, by inability to multiply normally. The failure of an rll mu- 

 tant to mature in K can be overcome by the presence of a wild-type phage in the 

 same cell. This could be understood if the function of the region in the wild-type 

 "chromosome" were to control the production of a substance or substances needed 

 for reproduction of this phage in K cells. 



The phenotypic test for "pseudo-allelism" leads to the division of the region into 

 two functionally distinguishable segments. These could be imagined to affect two 

 necessary sequential events or could go to make up a single substance the two parts 

 of which must be unblemished in order for the substance to be fully active. For 

 example, each segment might control the production of a specific polypeptide 

 chain, the two chains later being combined to form an enzyme. While it is not 

 known whether this sort of picture is applicable, a model of this kind is capable of 

 describing the observed properties of the rll mutants. The map position of a mu- 

 tation would localize a change in the region (and also in the "enzyme" molecule), 

 the reversion rate would characterize the type of change involved in the genetic 

 material, and the degree of phenotypic effect would be an expression of the degree 

 of resultant change in the activity of the enzyme. A "leaky" mutant would be 

 one where this latter effect was small. While no obvious correlation has yet been 

 observed among these three parameters of rll mutants, one may well show up 

 upon more exhaustive study. 



"Clustering" of similar mutants separable by crossing-over has been observed for 

 several characters in phage by Doermann and Hill and appears to represent the rule. 

 This may well be the rule in all organisms, simply because functional genetic units 

 are composed of smaller recombinational and mutational elements. One would 

 expect to see this effect more readily in phage because the probability of recom- 

 bination per unit of hereditary material is much greater than for higher organisms. 



By extension of these experiments to still more closely linked mutations, one may 

 hope to characterize, in molecular terms, the sizes of the ultimate units of genetic 

 recombination, mutation, and "function." Our preliminary results suggest that 

 the chromosomal elements separable by recombination are not larger than the 

 order of a dozen nucleotide pairs (as calculated from the smallest non-zero recom- 

 bination value) and that mutations involve variable lengths which may extend 

 over hundreds of nucleotide pairs. 



In order to characterize a unit of "function," it is necessary to define what func- 

 tion is meant. The entire rll region is unitary in the sense that mutations any- 

 where within the region cause the rll phenotype. On the basis of phenotype tests 

 of irans configuration heterozygotes, this region can be subdivided into two func- 

 tionally separable segments, each of which is estimated to contain of the order of 

 4 X 10^ nucleotide pairs. If one assumes that each segment has the "function" of 

 specifying the sequence of amino acids in a polypeptide chain, then the specification 

 of each individual amino acid can as well be considered a unitary function. It 



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