Molecular genetic analyses of the soybean mosaic virus NIa proteinase

Recombinant DNA molecules containing cDNA to a soybean mosaic virus (SMV) RNA genome were constructed and partial nucleotide sequences determined for two cDNA inserts, pSMV-34 and pSMV-35. Comparison of the predicted amino acid sequence encoded by the pSMV-34 cDNA insert to other potyvirus protein sequences revealed extensive homology with the region of the genome encoding the NIa proteinase, with conservation of the amino acids proposed to form the catalytic triad of the active site. Cell-free transcription and translation of the cloned cDNA sequence containing the NIa open reading frame and flanking sequences revealed that NIa proteinase sequences, which were expressed as part of a high Mr precursor, were able to undergo proteolytic processing. Alteration of the codon for one of the putative active site residues by site-directed mutagenesis eliminated processing and resulted in the accumulation of a high Mr precursor. Based on predicted amino acid sequences at five putative cleavage sites within the SMV polyprotein, a consensus SMV NI a proteinase cleavage sequence of Glu/Asn-Xaa-Val-Xaa-Xaa-Gln decreases Gly/Ser was proposed. The SMV NIa proteinase and its putative cleavage sites maintained motifs found in other potyviruses.