Protein Bound to the Genome RNAs of Cowpea Mosaic Virus

Evidence is presented for a previously undiscovered protein that is associated with the virion of cowpea mosaic virus (CPMV). It co‐purified with the viral RNA. The bond between this genomeassociated protein and the viral RNA is stable to treatments that generally disrupt non‐covalent bonds, including heating in sodium dodecylsulfate solution and extraction with phenol. The protein has been characterized by peptide mapping and gel electrophoresis; its molecular weight was estimated to be ∼5000 from mobility in gels. One of the zones seen on autoradiographs of gel electrophoresis profiles of ribonuclease T1‐digested 32P‐labeled CPMV RNAs was designated oligonucleotide‐proteinT1. Only this zone was found when the digestion was repeated using iodinated RNA. This zone was proteasesensitive, while other T1‐generated oligonucleotides were not. The oligonucleotide‐proteinT1 was not a substrate for polynucleotide kinase, though other ribonuclease‐T1‐genearted oligonucleotides did accept a 5′‐terminal phosphoryl group under the same conditions. Since the intact CPMV RNAs also were not substrates for polynucleotide kinase and lack any of the usual 5′‐ends [Klootwijk, J., Klein, I., Zabel, P. & van Kammen, A., Cell, 11, 73–82 (1977)], this substance is probably (an apparently covalent) protein‐oligonucleotide complex derived from the 5′‐end of CPMV RNAs.