Plant viruses with circular single-stranded DNA

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) mosaic3-5, 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 ribonuclease 1. 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. © 1977 Nature Publishing Group.