A clock- and light-regulated gene that links the circadian oscillator to LHCB gene expression

We have identified and characterized a novel tobacco gene, called ZGT (from the Chinese phrase zhong guang tiaokong, or clock and light controlled), t hat is regulated by the circadian clock and light. ZGT transcripts have alt ernate forms that are differentially expressed in different tissues. ZGT is expressed rhythmically in light/dark cycles and in constant light. Constit utive expression of ZGT sustains the expression of the clock-controlled LHC B1*1 gene in constant darkness, when it would normally dampen, but does not affect LHCB1*1 expression in constant light. ZGT expression is induced rap idly by light, and overexpression of ZGT increases the sensitivity of the c ircadian oscillator to brief light pulses. The ZGT promoter includes a G-bo x motif that is found in many light-regulated promoters in plants and is th e same as the E box described for rhythmically regulated promoters of anima l circadian clock genes. The ZGT promoter also includes "evening element" m otifs that are correlated with circadian control of plant genes. We postula te that light- and clock-regulated expression of ZGT acts as a coupling age nt between the central circadian oscillator and rhythmic LHCB1*1 expression and that it may function as a component in plant phototransduction pathway s.