Rice yellow mottle sobemovirus (RYMV) is responsible for the yellow mottle disease on rice in Africa. The expression and function of the protein P1 (17.8 kDa) encoded by the first open reading frame (ORF) of RYMV was investigated. Using an antibody raised against purified P1, two proteins with apparent molecular masses of 18 and 19 kDa were identified in in vitro translation reactions of transcripts of the full-length cDNA of RYMV. Likewise, gene products with similar molecular mass were detected in inoculated and systemically infected rice leaves and in infected rice protoplasts. A mutant from which ORF1 nucleotides 88 to 547 were deleted and a frameshift mutant that resulted in truncation of 83 amino acids from the C terminus of P1 were incapable of replicating in protoplasts. In contrast, a mutant that does not express P1 due to a mutation at the initiation codon replicated efficiently in protoplasts but at a reduced level (about 0.5- to 2-fold less) compared to replication of wild-type RNA. None of these mutants caused systemic infection in rice plants. Transgenic rice plants that express P1 complemented the initiation codon mutant, but not the deletion mutants, and produced systemic infection. These experiments demonstrate that P1 of RYMV is dispensible for virus replication, although nucleotide deletions or additions in ORF1 are apparently lethal for virus replication. Furthermore, P1 of RYMV is required for the infection of plants and is important for virus spread.