AMINO-ACIDS IN THE CAPSID PROTEIN OF TOMATO YELLOW LEAF CURL VIRUS THAT ARE CRUCIAL FOR SYSTEMIC INFECTION, PARTICLE FORMATION, AND INSECT TRANSMISSION

A functional capsid protein (CP) is essential for host plant infection and insect transmission in monopartite geminiviruses. We studied two defective genomic DNAs of tomato yellow leaf curl virus (TYLCV), Sic a nd SicRcv. Sie, cloned from a field-infected tomato, was not infectiou s, whereas SicRcv, which spontaneously originated from Sie, was infect ious but not whitefly transmissible. A single amino acid change in the CP was found to be responsible for restoring infectivity. When the am ino acid sequences of the CPs of Sic and SicRcv were compared with tha t of a closely related wild-type virus (TYLCV-Sar), differences were f ound in the following positions: 129 (P in Sic and SicRcv, Q in Sar), 134 (Q in Sic and Sar, H in SicRcv) and 152 (E in Sic and SicRcv, D in Sar). We constructed TYLCV-Sar variants containing the eight possible amino acid combinations in those three positions and tested them for infectivity and transmissibility. QQD, QQE, QHD, and QHE had a wild-ty pe phenotype, whereas PHD and PHE were infectious but nontransmissible . PQD and PQE mutants were not infectious; however, they replicated an d accumulated CP, but not virions, in Nicotiana benthamiana leaf discs . The Q129P replacement is a nonconservative change, which may drastic ally alter the secondary structure of the CP and affect its ability to form the capsid. The additional Q134H change, however, appeared to co mpensate for the structural modification. Sequence comparisons among w hitefly-transmitted geminiviruses in terms of the CP region studied sh owed that combinations other than QQD are present in several cases, bu t never with a P129.