The double-stranded RNA in Trichomonas vaginalis may originate from virus-like particles

A linear 5.5-kilobase double-stranded RNA, identified in many strains and isolates of the parasitic protozoan Trichomonas vaginalis in a previous study, is found largely intact in ribonuclease-treated homogenates of the parasite. It can be pelleted with membranes from the homogenate at 12,500 X g and further purified in CsCl buoyant density-gradient centrifugations. The purified sample contains the double-stranded RNA as well as one major protein with an estimated molecular mass of 85 kDa in NaDodSO4/PAGE. Electron microscopic examinations indicated the presence of icosahedral virus-like particles of 33-nm diameter in the purified preparation. The exact location of the virus in T. vaginalis is not clear, except that it is not found in the nuclear fraction and is probably membrane-bound. No free virus can be recovered from the culture medium of T. vaginalis, and no successful infection of virus-free T. vaginalis strains by purified virus has yet been accomplished. There is no viral genomic sequence identifiable in host DNA. So far as we know, it is the first time a double-stranded RNA virus has been identified in a protozoan.