PRESENCE OF DOUBLE-STRANDED-RNA AND VIRUS-LIKE PARTICLES IN PHAFFIA-RHODOZYMA

Four double-stranded RNA (dsRNA) molecules were isolated from Phaffia rhodozyma UCD 67-385. Their molecular sizes were approximately 4.3, 3.1, 0.9 and 0.75 kilobase pairs (kbp) as determined by agarose-gel electrophoresis and they were designated as L, M, S-1 and S-2, respectively. By differential centrifugation in sucrose gradients, these dsRNAs copurified with isometric virus-like particles 36 nm in diameter. A cured strain. UV-S2, lacking the S-2-dsRNA was obtained from P. rhodozyma UCD 67-385 by ultraviolet (UV) light treatment. UV-S2 strain contains identical virus-like particles to those from the wild-typo strain, as determined by electron microscopy, suggesting that the S-2-dsRNA was not essential for the expression of mycovirus structural polypeptides. On the other hand, both the UCD 67-385 and UV-S' strains were able to kill P. rhodozyma UCD 67-383, a strain without dsRNAs. These results suggest that the dsRNA molecules also encode a killer system. Finally, the UV-S' strain maintains killer ability, which suggests that S-2-dsRNA is not involved in the killer phenotype expression.