Split genes in chloroplasts
Split genes for ribosomal RNA (rRNA), transfer RNAs and some proteins have also been reported in the chloroplast genomes of several plants including Chlamydomonas and Nicotiana. Introns found in chloroplast genes can be classified into three groups on the basis of intron boundary sequences, (i) Group I introns (e.g. in tniL)can be folded in a secondary structure similar to self splicing rRNA precursor of Tetrahymena. These can be removed either by self splicing or by a 'maturase' (as in cytochromc b and cytochrome oxidasc mRNA precursors), (ii) Group II introns (e.g. majority of genes including trnA and trnI)can be folded into a complex secondary structure (as in introns of mitochondrial genes for cytochrome oxidase in maize and yeast), (iii) Group III introns (e.g. trnG, trnK, trnV, rpl2, rps12, rps16, etc.) have conserved sequences at their borders (GTGCGNY at 5' end, and ATCNRYY(N)YYAY at 3' end), similar to those in the eukaryotic nuclear genes.