Plant viruses with circular single-stranded DNA

Abstract

SMALL quasi-isometric particles, mostly occurring in pairs, have been found in, and purified from extracts of plants infected by maize streak¹, beet curly top², tomato golden mosaic³, euphorbia mosaic³, bean golden (yellow) mosaic^3–5, cassava latent⁶ and cassava brown streak viruses⁶. Individual particles are 15–20 nm in diameter—unusually small for a virus—and in electron micrographs many of the individual particles in the pairs have a five-sided outline in which the contiguous sides seem longer than the others. The pairs of particles of maize streak and cassava latent viruses have sedimentation coefficients of about 76S (refs 1 and 7) and preparations of each yield a single polypeptide species, estimated at about 28,000 and 34,000 daltons respectively⁷. Their nucleic acid can be resolved into two components by electrophoresis⁷, and in maize streak virus it has been identified tentatively as RNA on the basis of sensitivity to ribonuclease¹. We now report, however, that the nucleic acids of cassava latent and maize streak viruses consist of single-stranded, predominantly circular DNA of molecular weight less than 10⁶.