Tobacco streak virus (TSV) RNA and alfalfa mosaic virus (AIMV) RNA will replicate only if a few copies of their coat proteins are bound to the RNA. To understand this phenomenon experiments were performed to find unique features of the TSV and AIMV subunits. Atomic absorption analysis showed that TSV and AIMV contained substantial quantities of zinc in native virions (approximately one zinc atom per four protein subunits in TSV and one zinc atom per two protein subunits in AIMV), while other plant viruses tested did not. Treatment of TSV with a zinc-extracting reagent resulted in partial degradation of all the TSV nucleoprotein components, although the top component was most effected. The sequence (Cys X2 Cys X10 Cys X2 His) was found between residues 28 and 45 in the TSV primary structure and it is similar to a sequence found in several nucleic acid-binding, gene-regulatory proteins, most notably transcription factor IIIA from Xenopus laevis. TSV subunits were found to be extensively crosslinked within the virions. TSV and AIMV contain sequences rich in basic residues in the amino-terminal portion of the subunit (residues 51 to 72 in TSV and 1 to 26 in AIMV) and helical predictions suggested modes of protein-nucleic acid interactions in these regions similar to those proposed for histones. Two potential sites for glycosylation were identified near the amino terminus of the TSV sequence. Controlled treatment of TSV with trypsin removed 87 residues from the amino terminus and produced a monomer of cleaved protein, as analysed by SDS-PAGE.(ABSTRACT TRUNCATED AT 250 WORDS)