Cauliflower mosaic virus (CaMV) DNA exists under different topological forms in infected plants. First, the population of encapsidated CaMV DNA molecules appears heterogeneous when analysed by gel electrophoresis. The electron microscopic study reported here reveals that CaMV virion DNA contains simple and multiple topological knots. Second, a supercoiled DNA form never found in virions exists as a chromatin‐like nucleoprotein complex with nucleosome subunits in the nuclei of infected leaves. The compaction ratio of the minichromosomes is compatible with the nucleosomal structure, the number of nucleosomes (41.0 +/‐ 2.5) is in keeping with the length of the viral genome.

SUMMARYElectron microscopy of purified particles of a virus found in Dig#aria sanguinalis from Vanuatu (formerly New Hebrides) indicated that it is a geminivirus. Preparations of virus particles contained one coat protein of tool. wt. about 27 500 and circular and linear single-stranded DNA about 2350 nucleotides in length. In thin sections of infected cells, geminate particles were found in crystalline arrays both in nuclei and in the cytoplasm. The virus is serologically related to maize streak virus but differs from it with a serological differentiation index of 3.A striate mosaic disease of Digitaria sanguinatis in Vanuatu (formerly New Hebrides) was described some years ago (Dollet & De Taffin, 1979), but the causal agent was not identified. In this paper, we show that the diseased plants contain geminate virus particles (Francki et al., 1985; Harrison, 1985) that are serologically related to maize streak virus (MSV) (Bock et al., 1974).Infected D. sanguinalis plants were collected in the field and vegetatively propagated in an insect-proof glasshouse. Healthy D. sanguinalis was grown from seed in sterilized soil in the same glasshouse.Pieces of diseased leaf were fixed in glutaraldehyde and osmium tetroxide and embedded in epoxy resin (Dollet et al., 1983). Sections were stained in uranyl acetate and lead citrate. Linear aggregates of geminate particles (16 to 21 nm) were observed in the nuclei of companion and phloem parenchyma cells, often in large amounts (Fig. 1). Similar arrays of particles were also found in intact areas of cytoplasm, in cells in which nuclei appeared to be intact. Among other geminiviruses, this unusual location has been reported only for chloris striate mosaic virus (CSMV) (Francki et al., 1979). Virus particles were purified by a method derived from that of Larsen & Duffus (1984). Frozen leaves were ground in 0-1 M-potassium phosphate buffer pH 7, containing 10 mM-EDTA, 10 mM-sodium diethyldithiocarbamate and 20 mM-Na2SO3. The sap was treated with 1% Triton X-100 and clarified with 1/10 vol. chloroform/butanol (1:1, v/v). Virus particles were concentrated with 12% polyethylene glycol and 0.2 M-NaC1 and a cycle of differential centrifugation. The pellets were dissolved in 0.1 M-phosphate buffer pH 7 containing 10 mM-EDTA, and the virus preparation ultracentrifuged in a 10 to 40% sucrose density gradient prepared in the same buffer. The virus-containing band was concentrated by ultracentrifugation, and the sucrose density gradient centrifugation was then repeated. The yield was about 6 mg of virus per kg of fresh leaf tissue (a relatively large amount for a geminivirus), estimated assuming that Almg/ml is 7"7 (Goodman & Bird, 1978). Most particles in the purified virus ~ ~260nm, I cm preparations were geminate (Fig. 2), and the remaining few were single. The methods of 0000-6920 © 1986 SGM

No abstract