392 S. GARD AND O. MAAL0E 



permeable to water than to tlie solute. When the pressure drops, the slow 

 diffusion mto the medium of solute molecules, and the simultaneous rapid 

 diffusion into the phage of water molecules apparently create a diffusion 

 pressure high enough to open pores in the phage membrane and allow the 

 DNA to escape (Anderson, 1953). 



It has been found that the phage particle can exist in a different state in 

 which it is not susceptible to osmotic shock. If phage T6 is equilibrated with 

 2-5 M NaCl at 55° C, it is resistant to osmotic shock, but if the suspension is 

 cooled before being diluted, the phage returns to the sensitive state at a rate 

 which depends strongly on the temperature at which it is held. At 0°C. the 

 half-time for reversion to sensitivity is about 24 hours ! In the resistant state, 

 the phage membrane is assumed to be sufficiently permeable to solute mole- 

 cules to prevent inactivation upon dilution mto water (Anderson, 1953). 



Phage T6, in the sensitive state, is inactivated directly in concentrated 

 sucrose solutions. Apparently the membrane is impermeable to the sugar and 

 inactivation residts from dehydration. In the resistant state, the phage is 

 stable in sucrose solutions but can be shocked by rapid dilution into water 

 (Anderson et at., 1953). 



The reversible transition between the sensitive and the resistant state has 

 a very high temperature coefficient, to which corresponds an entropy change 

 of about 250 cal./degree (Anderson et al., 1953). It is interesting to compare 

 these two states with the active and inactive states of phage Tl (see p. 390); 

 at relatively low temperature, the transition from the resistant to the sensitive, 

 as well as from the inactive to the active state, is remarkably sloiv. In fact, it is 

 the sluggishness of these reactions which first allowed the recognition of two 

 states. 



Phage ghosts adsorb onto sensitive bacteria almost as well as do normal 

 phages; Herriott (1951) has demonstrated that the adsorption of a ghost may 

 be enough to kill a bacterium. This shows that killing must be associated 

 with a very early step in infection, since it does not depend on phage DNA 

 entering the cell. 



IV. Chemical Agents 

 S. Gard 



A. Chemical Alterations of the Virus Particle not associated with Loss 



of Infectivity 



The first systematic attempts to induce specific chemical alterations of the 

 TMV proteui were reported by Scliramm and Miiller in 1940. These authors 

 treated the virus with ketene or phenyl isocyanate. They found that the 

 resulting acetylated or phenylureido viruses retained their infectivity even 



