Strain i 118 of tobacco mosaic virus in tobacco plants kept at 35° exists mainly as free virus RNA and insoluble virus protein but also forms a few intact virus particles. Buffer extracts of infected leaves have, therefore, very little infectivity. Similar extracts from plants inoculated with a mixture of i 118 and the type strain tested on plants in which only i 118 gives symptoms are very much more infective (40 to 500 times). The increase in the i 118 infectivity of leaf extracts is even greater when the leaves are infected with the Nigerian cowpea virus instead of the type strain. The cowpea virus is a strain of tobacco mosaic virus only slightly serologically related to i 118. Neutralization of infectivity tests, using homologous and heterologous antisera, showed that the increased infectivity of i 118 in dual infections is caused by the nucleic acid of i 118 being incorporated in protein of Nigerian cowpea virus.


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