476 J. BRACKET 



multiplication; (2) the finding that pentosenucleoproteins, presumably in 

 the form of microsomes, play an important part in embryonic develop- 

 ment; and (3) the presence of large amounts of PNA in all cells in which 

 considerable protein synthesis takes place. These three different aspects of 

 the same major problem, i.e., the role of PNA in protein synthesis, will be 

 discussed in the present chapter. 



I. The Role of PNA in Plant Viruses 



This question has recently been ably reviewed by Markham,^ who has 

 made important personal contributions in this field. 



1. The Composition of Plant Viruses 



All the plant viruses which have been purified and, in many instances, 

 crystallized, contain large amounts of PNA, ranging from 6% (tobacco 

 mosaic virus) up to 35 % (turnip yellow mosaic virus) , in close association 

 with protein. The fact that such relatively simple nucleoproteins are able 

 to reproduce themselves and are thus endowed with genetic continuity is 

 of the utmost importance : plant viruses are ideal material for the study of 

 the synthesis of specific proteins as well as for the solution of fundamental 

 genetic problems. But we shall deal here only with the significance of PNA 

 in plant virus multiplication. 



2. The Importance of PNA in Plant Virus Multiplication 

 a. Abnormal Proteins from Virus- Infected Plants 



In their important studies on turnip yellow mosaic virus, Markham and 

 Smith^ were able by ultracentrifugation to separate two distinct com- 

 ponents from crystalline preparations of the virus: both contained sero- 

 logically identical proteins, but only the more rapidly sedimenting com- 

 ponent contained PNA in addition and proved infective When the virus 

 nucleoprotein was introduced into a plant, about 40 % of the particles pro- 

 duced were devoid of PNA and incapable of multiplication. These findings 

 led Markham^ to the very important conclusion that "there is some evi- 

 dence that the nucleic acid is in fact the substance controlling virus 

 multiplication." 



Markham and Smith V findings were soon extended: in 1952, Takahashi 

 and Ishii^'' reported the isolation from mosaic-diseased tobacco leaves of 

 an abnormal protein which could be obtained by electrophoresis and which 



1 R. Markham, ^nd Symp. Soc. Gen. Microbiol, p. 85 (1953); Advances in Virus Re- 

 search 1, 315 (1953); Progr. in Biophys. and Biophys. Chem. 3, 61 (1953). 



2 R. Markham and K. M. Smith, Parasitology 39, 330 (1949). 



' W. N. Takahashi and M. Ishii, Phytopathology 42, 690 (1952). 

 * W. N. Takahashi and M. Ishii, Am. J. Botany 40, 85 (1953). 



