ATGRP7, A NUCLEAR RNA-BINDING PROTEIN AS A COMPONENT OF A CIRCADIAN-REGULATED NEGATIVE FEEDBACK LOOP IN ARABIDOPSIS-THALIANA

The endogenous clock that drives circadian rhythms is thought to commu nicate temporal information within the cell via cycling downstream tra nscripts, A transcript encoding a glycine-rich RNA-binding protein, At grp7, in Arabidopsis thaliana undergoes circadian oscillations with pe ak levels in the evening, The AtGRP7 protein also cycles with a time d elay so that Atgrp7 transcript levels decline when the AtGRP7 protein accumulates to high levels, After AtGRP7 protein concentration has fal len to trough levels, Atgrp7 transcript starts to reaccumulate. Overex pression of AtGRP7 in transgenic Arabidopsis plants severely depresses cycling of the endogenous Atgrp7 transcript, These data establish bot h transcript and protein as components of a negative feedback circuit capable of generating a stable oscillation, AtGRP7 overexpression also depresses the oscillation of the circadian-regulated transcript encod ing the related RNA-binding protein AtGRP8 but does not affect the osc illation of transcripts such as cab or catalase mRNAs, We propose that the AtGRP7 autoregulatory loop represents a ''slave'' oscillator in A rabidopsis that receives temporal information from a central ''master' ' oscillator, conserves the rhythmicity by negative feedback, and tran sduces it to the output pathway by regulating a subset of clock-contro lled transcripts.