Positional cloning of the hcf109 (high chlorophyll fluorescence ) mutation in Arabidopsis has identified a nucleus-encoded, plastid-localized release factor 2-like protein, AtprfB, indicating that the processes of translational termination in chloroplasts resemble those of eubacteria. Control of atprfB expression by light and tissues is connected to chloroplast development. A point mutation at the last nucleotide of the second intron causes a new splice site farther downstream, resulting in a deletion of seven amino acid residues in the N-terminal region of the Hcf109 protein. The mutation causes decreased stability of UGA-containing mRNAs. Our data suggest that transcripts with UGA stop codons are terminated exclusively by AtprfB in chloroplasts and that AtprfB is involved in the regulation of both mRNA stability and protein synthesis. Furthermore, sequence data reveal a +1 frameshift at an internal in-frame TGA stop codon in the progenitor prfB gene of cyanobacteria. The expression pattern and functions of atprfB could reflect evolutionary driving forces toward the conservation of TGA stop codons exclusively in plastid genomes of land plants.