The b 1 protein, produced in leaves of Nicotiana tabacum cv. Xanthi-nc following infection with tobacco mosaic virus, has been purified to homogeneity by a procedure which involves gel chromatography and absorption on to DEAEcellulose. One gel chromatography step was sufficient when the procedure was applied to leaf extracts made in an acid buffer, whereas two were necessary with extracts made at pH 8. The final product migrates as a single protein band on electrophoresis in both acrylarnide and SDS-acrylamide gels. Its tool. wt. is estimated to be 15o0o by electrophoresis and 142oo by ultracentrifugation. Amino acid analysis suggests that it contains about I36 residues of which 39 are potentially acidic, 13 basic and I6 aromatic. The absorbance coefficient A2s0~m is estimated to be I8. 9. No evidence was found for the presence of a nucleotide component
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