RAPPAPORT, SIEGEL AND WILDMAN II 



and extracellular virus is maintained for about 2 hours, after which the host- 

 virus complex becomes more and more resistant to the effects of UV. 



The change in slope 2 hours after infection may reflect the time necessary for 

 deeper cellular penetration of the virus particle, where it then becomes partly 

 shielded from the full dose of UV by other UV-absorbing materials, or, prior to 

 multiplication, the intracellular virus may undergo some change in form which 

 is more resistant to inactivation by ultraviolet light. As the time between 

 inoculation and irradiation is further increased, the shape of the inactivation 

 curve becomes 'multi- target' in nature. The first indication of virus multiplica- 

 tion is apparent after 7 hours of host-virus association. The 7-hour curve is a 

 theoretical survival curve for three particles within a cell, while the points 

 which fit the line are experimental. Nine hours after infection the curves sug- 

 gest that between five and eight particles, each independently capable of main- 

 taining the infection, are now present. 



On closer examination, it was found that the infectious unit passed through 

 three clearly defined phases of UV sensitivity before multiplication occurred. 

 N. glulinosa leaves were irradiated with a constant dose of UV (90 sec.) at dif- 

 ferent times after virus inoculation and the survival of the infectious centers 

 ascertained. The experiments were carried out at two different temperatures, the 

 results of which are recorded in figure 2. 



Four distinct step-like phases in the UV sensitivity of infectious centers are 

 represented. The first three phases are the following: an initial phase of constant 

 UV sensitivity; a second phase, showing a gradual increase in resistance to UV 

 damage, and a third phase, marked by a plateau of little changing sensitivity. 

 Up to and including this third phase, the slopes of the survival curves as 

 seen in figure i are exponential, so that whatever these changes in UV sensitivity 

 reflect, they are stiU expressions of only a single infectious unit. The fourth 

 phase displays a rapid rise in UV resistance which is correlated with the ap- 

 pearance of the 'multi-target' curve. Such resistance depends primarily on the 

 number of particles present in each infectious center. The timing of the phases 

 is highly temperature dependent. It takes almost twice the time to reach the 

 fourth or multiplicative phase with plants kept at 20°C than it does with the 

 same material at 3o°C. The qualitative features of the curves nonetheless re- 

 main unchanged. 



The meaning of the first three preparatory phases is obscure, but some addi- 

 tional information, obtained from other TMV strains in radiation studies, sug- 

 gest that the changes in UV resistance are related directly to the virus state, 

 rather than associated with any change in the sensitivity of the host cell. 

 Consequently, the final slopes of the 'multi-target' curves (see figure i) assume 

 an added significance. Their shallowness as compared to the //; vitro and first 

 2-hour in vivo curves suggests that the intracellular reproducing particles may 

 be different in some way from the in vitro rod. This notion concerning a dif- 



