Peptide chain release factor 1 directs the termination of translation in response to the peptide chain termination codons UAG and UAA in mitochondria. Mostly expressed in seedlings, stems and adult plants, and, to a lower extent, in siliques (PubMed:17450416). Barely detected in etiolated seedlings and roots (PubMed:17450416). Methylation increases the termination efficiency of RF1. Belongs to the prokaryotic/mitochondrial release factor family.