A trans-acting factor that binds to a GT-motif in a phytochrome gene promoter.

The regulatory photoreceptor, phytochrome, controls the expression of numerous genes, including its own phyA genes, which are transcriptionally repressed in response to light. Functional analysis of a rice phyA gene promoter, by means of microprojectile-mediated gene transfer, indicates that a GT motif, GCGGTAATT, closely related to elements in the promoters of a number of other light-regulated genes, is critical for expression. Partial complementary DNA clones have been obtained for a rice nuclear protein, designated GT-2, that binds in a highly sequence-specific fashion to this motif. Mutational analysis shows that the paired G's are most crucial to binding. GT-2 has domains related to certain other transcription factors. Northern blot analysis shows that GT-2 messenger RNA levels decline in white light although red and far red light pulses are ineffective.