Protozyme: emerging evidence in nature.

The VMA1 locus in Saccharomyces cerevisiae contains a nested genetic element, the VDE gene, and expresses two functional proteins. A single VMA1 translational product seems to catalyze a self protein splicing in which an internal domain is excised out to produce a site-specific DNA endonuclease and the N- and C-domains are ligated by a transpeptidation reaction to yield a catalytic subunit of the vacuolar ATPase. Accumulating evidence in the past few years suggests that the VMA1 locus encodes a protozyme (for protos en zyme) which has dual roles in life as a protean catalyst for self protein splicing and for self gene homing. Four protozymes that share a common mechanism in protein splicing have been found in four organisms covering three major phylogenic trees.