A new superfamily of putative NTP-binding domains encoded by genomes of small DNA and RNA viruses.

Statistically significant similarity was revealed between amino acid sequences of NTP-binding pattern-containing domains which are among the most conserved protein segments in dissimilar groups of ss and dsDNA viruses (papova-, parvo-, geminiviruses and P4 bacteriophage), and RNA viruses (picorna-, como- and nepoviruses) with small genomes. Within the aligned domains of 100-120 amino acid residues, three highly conserved sequence segments have been identified, i.e. 'A' and 'B' motifs of the NTP-binding pattern, and a third, C-terminal motif 'C', not described previously. The sequence of the 'B' motif in the proteins of the new superfamily is unusually variable, with substitutions, in some of the members, of the Asp residue conserved in other NTP-binding proteins. The 'C' motif is characterized by an invariant Asn residue preceded by a stretch of hydrophobic residues. As the new superfamily included a well studied DNA and RNA helicase, T antigen of SV40, helicase function could be tentatively assigned also to the other related viral putative NTP-binding proteins. On the other hand, the possibility of different and/or multiple functions for some of these proteins is discussed.