AN ENZYME SIMILAR TO ANIMAL TYPE-II PHOTOLYASES MEDIATES PHOTOREACTIVATION IN ARABIDOPSIS

The important issue of photoreactivation DNA repair in plants has beco me even more interesting in recent years because a family of genes tha t are highly homologous to photoreactivating DNA repair enzymes but th at function as blue light photoreceptors has been isolated. Here, we r eport the isolation of a novel photolyase-like sequence from Arabidops is designated PHR1 (for photoreactivating enzyme). It shares little se quence similarity with either type I photolyases or the cryptochrome f amily of blue light photoreceptors. Instead, the PHR1 gene encodes an amino acid sequence with significant homology to the recently characte rized type II photolyases identified in a number of prokaryotic and an imal systems. PHR1 is a single-copy gene and is not expressed in dark- grown etiolated seedlings: the message is light inducible, which is si milar to the expression profile for photoreactivation activity in plan ts. The PHR1 protein complements a photolyase-deficient mutant of Esch erichia coli and thus confers photoreactivation activity. In addition, an Arabidopsis mutant that is entirely lacking in photolyase activity has been found to contain a lesion within this Arabidopsis type II ph otolyase sequence. We conclude that PHR1 represents a genuine plant ph otolyase gene and that the plant genes with homology to type I photoly ases (the cryptochrome family of blue light photoreceptors) do not con tribute to photoreactivation repair, at least in the case of Arabidops is.