EXPRESSION OF THE HELPER COMPONENT PROTEIN OF POTATO VIRUS-Y (PVY) INESCHERICHIA-COLI - POSSIBLE INVOLVEMENT OF A 3RD PROTEASE

Transmission of polyviruses by aphids depends on the presence of a vir us encoded helper-component protein (HC) that also exhibits protease a ctivity. HC was expressed in E. coli from two types of clones: a full- length cDNA clone of PVY and two 5' end clones containing the first th ree cistrons (3.6-3.7 kbp). The clones derived from the 5' end of PVY expressed HC of the size of the mature component. Other proteins react ing with antibodies to HC were also observed, and their sizes correspo nded with those of expected intermediates resulting from partial prote ase cleavage of the three-cistron polyprotein. On the other hand, the only detectable HC-related product of the full-length clone was a matu re-size HC. The presence of a third PVY protease among the first three cistrons is therefore suggested.