Group I-intron trans-splicing and mRNA editing in the mitochondria of placozoan animals.

Placozoa - the simplest known free-living animals - have been considered primitive, early diverging metazoans based on mitochondrial genome structure and phylogeny. Here we reanalyze placozoan mitochondrial DNAs, reported to include a highly unorthodox, fragmented and incomplete cox1 gene. We discover overlooked exons and split group I introns that mediate trans-splicing of the discontinuous placozoan cox1. Furthermore, we find that cox1 expression involves U-to-C editing, reconstituting an otherwise invariant, essential histidine involved in copper binding. These atypical features qualify placozoan mitochondrial gene and genome organization as derived rather than primitive. Whether the Placozoa diverged early or late during metazoan evolution remains unresolved by mitochondrial phylogeny.