Intron-exon organization and phylogeny in a large superfamily, the paralogous cytochrome P450 genes of Arabidopsis thaliana

The cytochrome P450 gene superfamily is represented by 80 genes in animal g enomes and perhaps more than 300 genes in plant genomes. We analyzed about half of all Arabidopsis P450 genes, a very large dataset of truly paralogou s genes. Sequence alignments were used to draw phylogenetic trees, and this information was compared with the inton-exon organization of each P450 gen e, We found 60 unique intron positions, of which 37 were phase 0 introns, O ur results confirm the polyphyletic origin of plant P450 genes. One group o f these genes, the A-type P450s, are plant specific and characterized by a simple organization, with one highly conserved intron, Closely related A-ty pe P450 genes are often clustered in the genome with as many as a dozen gen es (e.g., of the CYP71 subfamily) on a short stretch of chromosome. The oth er P450 genes (non-A-type) form several distinct clades and are characteriz ed by numerous introns, One such clade contains the two CYP51 genes, which are thought to encode obtusifoliol 14 alpha demethylase, The two CYP51 gene s have a single intron that is not shared with CYP51 genes from vertebrates or fungi, or with any other Arabidopsis P450 gene. Only a few of the Arabi dopsis P450 genes are intronless (e.g., the CYP710A and CYP96A subfamilies) . There was a relatively good correlation between intron conservation and p hylogenetic relationships between members of the P450 subfamilies. Gene org anization appears to be a useful tool in establishing the evolutionary rela tedness of P450 genes, which may help in predictions of P450 function.