INDEPENDENT EXPRESSION OF THE FIRST 2 TRIPLE GENE BLOCK PROTEINS OF BEET NECROTIC YELLOW VEIN VIRUS COMPLEMENTS VIRUS DEFECTIVE IN THE CORRESPONDING GENE BUT EXPRESSION OF THE 3RD PROTEIN INHIBITS VIRAL CELL-TO-CELL MOVEMENT

Cell-to-cell movement of beet necrotic yellow vein furovirus is contro lled by three slightly overlapping genes on RNA 2 called the triple ge ne block (TGB) encoding, in order, P42, P13, and P15, Synthesis of P42 is directed by subgenomic RNA 2sub(a) while synthesis of both P13 and P15 is probably directed by a dicistronic subgenomic RNA, 2sub,, For complementation experiments, each TGB protein gene was inserted into a ''replicon'' derived from viral RNA 3, In mixed infections, the repli cons expressing P42 and P13 complemented RNA 2 mutants defective in th e corresponding gene, A P15-containing replicon did not complement a P 15-defective RNA 2 but complementation was observed with a dicistronic replicon containing the P15 gene placed behind the P13 gene. In mixed infections with wild-type viral RNAs, the P15-containing replicon did not inhibit viral RNA replication in protoplasts but blocked local le sion formation on leaves,Infection of leaves was also inhibited by an RNA3-derived replicon containing the third TGB gene from another furov irus, peanut clump virus. The results are consistent with a model in w hich viral cell-to-cell movement requires production of appropriate re lative amounts of P13 and P15, and their expression from a dicistronic subgenomic RNA provides a mechanism for coordinating their synthesis.