RNA editing in hornwort chloroplasts makes more than half the genes functional.

RNA editing in chloroplasts alters the RNA sequence by converting C-to-U or U-to-C at a specific site. During the study of the complete nucleotide sequence of the chloroplast genome from the hornwort Anthoceros formosae, RNA editing events have been systematically investigated. A total of 509 C-to-U and 433 U-to-C conversions are identified in the transcripts of 68 genes and eight ORFs. No RNA editing is seen in any of the rRNA but one tRNA suffered a C-to-U conversion at an anticodon. All nonsense codons in 52 protein-coding genes and seven ORFs are removed in the transcripts by U-to-C conversions, and five initiation and three termination codons are created by C-to-U conversions. RNA editing in intron sequence suggests that editing can precede intercistronic processing. The sequence complementary to the edited site is proposed as a distant cis-recognition element.