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Relationships and Functions of Virus Double-stranded RNA in a P4 Killer Strain of Ustilago maydis

S. L. Shelbourn^1,

P. R. Day^2,

¹ Department of Pure and Applied Biology, Imperial College of Science and Technology, London SW7 2BB
and² Plant Breeding Institute, Cambridge CB2 2LQ, U.K.

Double-stranded RNA (dsRNA) from virus particles isolated from a P4 killer strain (isolate 77) of Ustilago maydis, which secretes a protein toxin of apparent M_r approx. 10000 (10K) was resolved by PAGE into eight segments with sizes (kbp) 6·7 (H1), 4·5 (H2), 3·2 (H3a), 3·1 (H3b), 2·6 (H4), 0·98 (M2-4), 0·92 (M3-4) and 0·36 (L1). A subculture, designated 77-3A, was found to have lost the H2 and M3-4 segments, but still produced virus particles and protein toxin. Northern hybridization showed that dsRNA segments M2-4 and L1 are related but no homology was detected between H1, H3a, H3b and H4, or between any of these H segments and M2-4 or L1. When individual denatured dsRNA segments were translated in vitro in a reticulocyte lysate system H1, H3a, H3b and H4 each gave rise to polypeptides in the M_r range 100K to 128K. Polypeptides of apparent M_r 108K and 128K, encoded by H3b, and 100K, encoded by H4, were immunoprecipitated by antiserum to purified virus particles from subculture 77-3A. In vitro translation of denatured M2-4 gave rise to polypeptides of apparent M_r 26K and 13K which were immunoprecipitated by antiserum to the secreted protein toxin. No in vitro translation product was detected from denatured L1.

Keywords: Ustilago maydis, killer proteins, double-stranded RNA, capsid polypeptide

Present address: University of Cambridge Clinical School, Department of Medicine, Addenbrooke's Hospital, Hills Road, Cambridge CB2 2QQ, U.K.

Present address: Cook College, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 080903, U.S.A.

Received 19 November 1987; accepted 2 February 1988.