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Sixteen groups of plant viruses are described. The type members and main characteristics of each group are listed. The extension of this system to other plant viruses is discussed, and it is recommended that only well studied viruses should be grouped. Names are proposed for twelve of the groups: bromovirus, carlavirus, caulimovirus, comovirus, cucumovirus, nepovirus, potexvirus, potyvirus, tobamovirus, tobravirus, tombusvirus, and tymovirus. We suggest that the value of these names should be tested by using them for an experimental period.

Crude sap from Moore barley infected with brome mosaic virus (BMV) and purified BMV produced two and sometimes three precipitation lines when diffused against an antiserum to purified virus in the Ouchterlony double-diffusion test. Antigens, which sedimented more slowly than infectious virus, were detected after density-gradient centrifugation at pH 7.0 and 8.0 of crude sap from BMV-infected plants and of BMV previously purified by density-gradient centrifugation, a result indicating that the virus dissociated during density-gradient centrifugation. The stability of crude and purified virus preparations decreased as pH was increased from pH 5.0 to 8.0, whether stability was measured by loss of infectivity or by the increase in concentration of breakdown products. Materials in boiled extracts of healthy barley stabilized the virus. The stabilization could not be detected at pH 5.0, but it became increasingly greater as pH was increased from pH 6.0 to 8.0. Purified virus dissociated into two subunits of uniform size. These subunits, antigens 1 and 2, were noninfectious; apparently they were nucleoproteins. Noninfectious antigens from above the infectious zone in density-gradient columns were concentrated by high speed centrifugation and, when analyzed by the Ouchterlony test, formed precipitation lines identical to those formed by purified virus. It is suggested on the basis of immunoelectrophoretic analysis of crude sap and purified virus preparations that the virus might have dissociated in the agar gel to produce these antigens.

Purified brome mosaic virus was stabilized at pH 7 by 10 −2 , 10 −3 , and 10 −4 M MgCl 2 and by 10 −2 M CaCl 2 , and at pH 6 by 10 −2 M MgCl 2 , the only concentrations tested. The virus was less stable at pH 7 in 0.1 M NaCl or 0.1 M potassium phosphate buffer than in 0.01 M buffer only, whereas at pH 6 it was more stable at the higher salt concentration. Sodium ascorbate, sodium sulfite, sodium ethylenediamine tetraacetate, and sodium oxalate decreased the stability of the virus at pH 7. The virus was more stable in tris(hydroxymethyl)aminomethane buffers than in phosphate. Healthy barley sap stabilized purified brome mosaic virus, but the extent of stabilization was not consistent from experiment to experiment. Infectivity assays were used to determine the concentration of surviving virus, but the stabilization by magnesium could also be shown by analytical density-gradient centrifugation. Frozen virus survived better in 0.01 M Tris-citrate pH 6.5 buffer with 0.001 M MgCl 2 and 0.1 M glycine and in 0.01 M acetate pH 6.0 with 0.001 M MgCl 2 and 0.1 M glycine than in other buffers tested.

The protein and RNA of bromegrass mosaic virus were separated from each other by dialysis of the virus against 1 M -calcium chloride. In this medium the virus is degraded, the RNA is quantitatively precipitated and there remains a supernatant solution of viral protein. The protein obtained in this way was soluble in various buffers in the pH range 5·5 to 6·5 and exhibited a single symmetrical sedimenting boundary in the ultracentrifuge. Its molecular weight was 40,000 ± 2,000 as determined by the Ehrenberg method of approach to sedimentation equilibrium. Amino acid analyses indicated that there were about 52 lysyl plus arginyl residues per molecular weight of 40,000. Tryptic hydrolysis of urea-denatured protein yielded 23 soluble peptides and an insoluble residue amounting to about 20% by weight of the intact protein. From the number of soluble tryptic peptides and an assumed number of tryptic peptides in the insoluble residue it was concluded that the molecular weight of the protein subunit of bromegrass mosaic virus was slightly greater than 20,000 and that the protein moiety obtained by treatment with 1 M -CaCl 2 was a relatively stable dimer of the subunit. Partial separation of the soluble peptides resulting from tryptic hydrolysis was achieved on a preparative scale by column chromatography.

Broad bean mottle virus can be degraded by dialysis against 1 M -CaCl 2 . The virus RNA is quantitatively precipitated together with some of the protein. Most of the virus protein, free of RNA, can be recovered from the supernatant liquid. The amino acid composition of the protein was determined. End group analyses were performed on the oxidized protein. The C-terminal amino acid was found to be alanine. No N-terminal amino group was found. The tryptic peptides of the oxidized protein were separated by a two-dimensional combination of paper chromatography and electrophoresis. Twenty-eight peptides were detected. From the number of tryptic peptides and the amino acid composition, the molecular weight of the protein subunit was calculated to be 20,500.

X-ray scattering data and electron microscope observations are presented for bromegrass mosaic virus. Its radial density distribution is obtained from the Fourier transform of the amplitudes of the scattered x-rays. The results indicate that the virus is 260 A in diameter, it has an almost empty central cavity which is about 80 A in diameter, and the regions occupied by RNA and protein are approximately equal in average density. Electron micrographs of negatively stained preparations also give an outside diameter of 260 A and indicate that there is a central region about 90 A in diameter into which uranyl acetate can penetrate. Positively stained preparations indicate that the nucleic acid is concentrated in a shell-shaped region which is in turn surrounded by a shell of protein. In order for the RNA and protein regions to have the same average electron density the RNA must have a hydration of 1.29 gm of water per gm of RNA and the protein must have a hydration of 0.24 gm of water per gm of protein.

Specific infectivity of cowpea chlorotic mottle virus (CCMV) declined with advancing age of infection in both whole plants and inoculated primary leaves. The decline began between 2 and 3 days after inoculation; 20–30 days later, purified preparations with similar amounts of virus nucleoprotein were only 1–5% as infective as the initial preparations. Specific infectivity was independent of host age, purification procedures, aggregation, and oxidative inhibitors. Specific infectivity increased when infected plants were transferred to 16°, presumably as a result of new particle formation; it declined rapidly at 33°.