PNZIP IS A NOVEL MESOPHYLL-SPECIFIC CDNA THAT IS REGULATED BY PHYTOCHROME AND A CIRCADIAN-RHYTHM AND ENCODES A PROTEIN WITH A LEUCINE-ZIPPER MOTIF

We isolated and characterized a novel light-regulated cDNA from the sh ort-day plant Pharbitis nil that encodes a protein with a leucine (Leu ) zipper motif, designated PNZIP (Pharbitis nil Leu zipper). The PNZIP cDNA is not similar to any other gene with a known function in the da tabase, but it shares high sequence homology with an Arabidopsis expre ssed sequence tag and to two other sequences of unknown function from the cyanobacterium Synechocystis spp. and the red alga Porphyra purpur ea, which together define a new family of evolutionarily conserved Leu zipper proteins. PNZIP is a single-copy gene that is expressed specif ically in led photosynthetically active mesophyll cells but not in oth er nonphotosynthetic tissues such as the epidermis, trichomes, and vas cular tissues. When plants were exposed to continuous darkness, PNZIP exhibited a rhythmic pattern of mRNA accumulation with a circadian per iodicity of approximately 24 h, suggesting that its expression is unde r the control of an endogenous clock. However, the expression of PNZIP was unusual in that darkness rather than light promoted its mRNA accu mulation. Accumulation of PNZIP mRNA during the dark is also regulated by phytochrome, since a brief exposure to red light in the middle of the night reduced its mRNA levels. Moreover, a far-red-light treatment at the end of day also reduced PNZIP mRNA accumulation during the dar k, and that effect could be inhibited by a subsequent exposure to red light, showing the photoreversible response attributable to control th rough the phytochrome system.