Nucleic Acids Research, 2008, Vol. 36, No. 22 6985 newly identiﬁed AtGRP8 feedback loop is interconnected with the AtGRP7 feedback loop previously identiﬁed as a circadian slave oscillator (9). The connection between the two regulatory circuits may serve to integrate input by diverse stimuli, and the crossregulation may ﬁne tune and balance the expression of both proteins. In the future it will be important to determine common target transcripts of AtGRP7 and AtGRP8 and to deter- mine the relative inﬂuence both proteins have on such downstream transcripts. CONCLUSION We show here that the clock-controlled RNA-binding protein AtGRP8 forms an interlocked post-transcriptional negative feedback loop with the AtGRP7 autoregulatory circuit. Both proteins negatively autoregulate and recipro- cally crossregulate by binding to their pre-mRNAs and promoting unproductive splicing coupled to degradation via the NMD pathway (Figure 8). Thus, we extend the examples of quantitative post-transcriptional control by Figure 7. Conservation of the retained part of the GRP introns. The nucleotide sequence corresponding to AtGRP8 exon I, exon II and the intron was aligned with the corresponding sequence of AtGRP7, SaGRP1 (8), BnGRP10 (46), NsGRP1a (47) and PhGRP2 (48). Figure 8. Model of the interlocked AtGRP7 and AtGRP8 feedback loops. Increasing AtGRP7 and AtGRP8 protein (depicted as ellipses) levels promote use of the cryptic intronic 50 splice sites, leading to unproductively spliced as_AtGRP7 and as_AtGRP8 transcripts that are degraded via the NMD pathway.