MOLECULAR ANALYSIS OF THE ESSENTIAL AND NONESSENTIAL GENETIC ELEMENTSIN THE GENOME OF PEANUT CHLOROTIC STREAK CAULIMOVIRUS

The DNA genome of caulimoviruses contains a set of essential genes: I (movement gene), IV (major capsid protein gene), V (reverse transcript ase gene), and VI (gene coding for a post-transcriptional activator of the expression of other virus genes). In peanut chlorotic streak caul imovirus (PCISV), three ORFs, A, B, and C, are located between genes I and IV. They are dissimilar to other caulimovirus ORFs. ORF VII of PC ISV is a homolog of ORF VII of soybean chlorotic mottle caulimovirus ( SoCMV), but is not similar to the nonconserved ORF VII in other caulim oviruses. The sequence complementary to a portion of tRNA(Met), though t to be essential for the priming of minus-strand DNA synthesis in cau limoviruses, is located within the coding sequence of ORF A. To explor e the functional significance of ORFs VII, A, B, and C, various mutati ons were engineered into an infectious DNA clone of PCISV. ORFs VII an d B are shown to be dispensable, while ORFs A and C are essential. ORF C is a possible functional equivalent of gene III in other caulimovir uses. Sequences within ORF A that are required for efficient priming o f minus-strand synthesis are likely to extend beyond the 12-bp tRNA-bi nding site. Complete deletion of ORF VII was correlated with severe sy mptoms, notably with the necrosis of apical meristems. significance of these observations for the understanding of replication and pathogene sis of plant pararetroviruses and for the improvement of caulimovirus- based expression Vectors is discussed. (C) 1995 Academic Press, Inc.