304 BACTERIOPHAGES 



certain mutants are rapid-lysers on E. coli B, but produce lysis 

 inhibition on E. coli K12 and other bacteria (Benzer, 1955, 1957). 



To produce lysis-inhibition, the primary infection must be 

 with r+ phage, but some r mutants will serve as the superinfecting 

 phage (Stent and Maal0e, 1953). Different r mutants of the 

 same phage vary greatly in physiological properties (Hershey, 

 1946b; Benzer, 1957). 



The r mutants form characteristic plaques, which explains 

 their importance in early genetic studies. One of these and 

 other mutants of T4 are illustrated in Figure 9. 



Cohen (1956) suggested that the r mutants, compared to their 

 parental phages, contain a larger amount of glucose as a con- 

 stituent of their nucleic acids. Sinsheimer (1956) failed to 

 confirm this, showing that the difference cannot be character- 

 istic of all r mutants. Different phages related to T2 do, how- 

 ever, differ in their content of DNA-glucose (Sinsheimer, 1956). 



The loci of all r mutations are widely spread throughout the 

 linkage structure of T4. A certain class of them, restricted to a 

 small region of the linkage structure, affect both the lysis-in- 

 hibiting property on E. coli B, and the growth potential in 

 lysogenic K12 (Benzer, 1955). The two phenotypic effects are 

 evidently not directly related, but neither one is understood. 

 The remarkable utility of the restricted host-range of the rll 

 mutants is discussed later in this chapter and in Chapter XVIII. 



The r mutation was probably first seen by Sertic (1929c) and 

 Demerec and Fano (1945). 



e. Mutations Affecting Plaque Type 



Almost any mutation in a phage is likely to alter the appear- 

 ance of its plaques. Mention has already been made of virulent 

 mutants of temperate phages, ; mutants, and mutants of de- 

 creased rate of adsorption. We describe here additional plaque- 

 type mutations the physiological basis for which has not been 

 investigated. 



That different phages produce plaques differing in appearance 

 was recognized very early. Examples were illustrated by Gratia 



