242 EOBERT L. SINSHEIMER 



increasing amounts of phage protein and of intact phage are made by 

 the time of lysis. Phage antigen does not begin to appear until 50 min- 

 utes after induction, but addition of ribonuclease at 25 or 40 minutes after 

 induction cannot prevent production, at the later time, of some antigen. 



An analysis of the total amount of phage antigen and total number 

 of infective phage particles made by the time of lysis, as compared to the 

 amount of antigen made by the time of addition of the ribonuclease, sug- 

 gests that infective phage particles can be formed only from the phage 

 protein made before the addition of the enzyme. Further analysis using 

 P 32 -labeling of the DNA suggests that phage antigen which is made at 

 a reduced rate after ribonuclease addition is apparently defective; it can 

 be assembled into phagelike particles but these cannot adsorb to the sus- 

 ceptible bacteria. This result may indicate an action of ribonuclease on 

 the centers producing the phage proteins. 



Measurements of the changes in the ultraviolet sensitivity of the phage- 

 bacterium complex during the course of phage development following ultra- 

 violet induction of E. coli C (X) have been reported by Benzer and Jacob. 205 

 Immediately following completion of the inducing dose of ultraviolet ra- 

 diation, the complex is more sensitive to additional ultraviolet radiation 

 than is the free X phage. 



In contrast, if E. coli C is infected with X phage, the ultraviolet sensi- 

 tivity immediately after infection is the same as that of the free phage. 

 With increasing time after induction, the ultraviolet sensitivity decreases 

 but more slowly than is observed following direct infection of E. coli C 

 with X phage. 



V. Nucleic Acids of Minute Bacteriophages 



While the existence of a class of minute bacteriophages has been known 

 for several decades, it is only within the past few years that any member 

 of this class has been seriously studied. It is now clear that these phages 

 comprise a group with quite distinct properties from those previously de- 

 scribed. 



The best known members of this group are S13 206 - 207 and 0X174. 208 ' 209 

 These phages are serologically related, have similar radiobiological prop- 

 erties, and appear from the early ultrafiltration and centrifuge studies to 

 be of similar size. As yet only 0X174 has been purified and subjected 

 to physical and chemical study. 209 



Phage 0X174 is but one-fortieth the size of the T2 phage and has about 



206 S. Benzer and F. Jacob, Ann. inst. Pasteur 84, 186 (1953). 



206 D. E. Lea and M. H. Salaman, Proc. Roy. Soc. B133, 434 (1946). 



207 S. A. Zahler, J. Bacleriol. 75, 310 (1958). 



208 V. Sertic and N. Bulgakov, Compt. rend. soc. biol. 119, 1270 (1935). 



209 R. L. Sinsheimer, J. Mol. Biol. 1, 37 (1959). 



