Leader proteinase of the beet yellows closterovirus: Mutation analysis of the function in genome amplification

The beet yellows closterovirus leader proteinase (L-Pro) possesses a C-term inal proteinase domain and a nonproteolytic N-terminal domain. It was found that although L-Pro is not essential for basal-level replication, deletion of its N-terminal domain resulted in a 1,000-fold reduction in RNA accumul ation. Mutagenic analysis of the N-terminal domain revealed its structural flexibility except for the 54-codon-long, 5'-terminal element in the corres ponding open reading frame that is critical for efficient RNA amplification at both RNA and protein levels.