Small cysteine-rich proteins of different groups of plant RNA viruses are related to different families of nucleic acid-binding proteins.

The 3'-terminal genes in genomic RNAs of four groups of plant positive strand RNA viruses (hordei-, furo-, tobra-, and carlaviruses) encode small proteins enriched in Cys residues. The arrangement of Cys and in some cases also His residues in these proteins is compatible with finger formation. A computer-assisted sequence comparison of viral Cys-rich proteins reveals no significant similarity between them. It is shown that the hordeivirus Cys-rich protein (17K) is related to a group of chloroplast tRNA intron-encoded proteins, whereas the respective furovirus protein (14K) is similar to E6 proteins of papillomaviruses. Somewhat less significant similarity, marked, however, by conservation of a number of positively charged residues, was observed between the sequences of tobravirus Cys-rich proteins and the basic domains of high mobility group chromatin proteins. It can be speculated that plant RNA virus Cys-rich proteins bind RNA and/or DNA and might be involved in regulation of virus and/or host genome expression. In the course of evolution these virus proteins may have originated from different nucleic acid-binding proteins.