Introns - Introns are DNA sequences within genes which disrupt protein coding regions termed exons. Introns and exons are contranscribed and the resultant precursor RNA spliced to remove introns. Four classes of introns have been recognized. One intron type is found in nuclear genes which encode proteins.

In contrast, the gene encoding tRNA-Lys(UUU) contains a 2.5 kbp Group" intron. An open reading frame within this gene's intron encodes a protein homologous to mitochondrial RNA maturases.This raises the interesting possibility that the putative maturase located in trnK could facilitate intron processing in plastids.

An even more remarkable observation was made recently with regard to rpsl2. This gene is encoded by three exons. The 5'-exon is located 28 kbp from the two 3'-exons and transsplicing is needed to include functional RNA. Transsplicing has also been reported for psaA in Chlamydomonas.