Transcriptional activation of DBP by hnRNP K facilitates circadian rhythm.

D-site albumin promoter binding protein (DBP) supports the rhythmic transcription of downstream genes, in part by displaying high-amplitude cycling of its own transcripts compared to other circadian clock genes. However, the underlying mechanism remains elusive. Here, we demonstrated that poly(C) motif within DBP proximal promoters, in addition to an E-box element, provoked the transcriptional activation through increased RNA polymerase 2 (Pol2) recruitment by inducing higher chromatin accessibility. We also clarified that heterogeneous nuclear ribonucleoprotein K (hnRNP K) is a key regulator that binds to the poly(C) motif on single-stranded DNAs in vitro. Chromatin immunoprecipitation further confirmed the expression-dependent and rhythmic binding of hnRNP K which was inhibited through its cytosolic localization mediated by time-dependent ERK activation. Knockdown of hnRNP K triggered low-amplitude mRNA rhythms in DBP and other core clock genes through transcriptional or post-transcriptional regulation. Finally, transgenic depletion of a Drosophila homolog of hnRNP K in circadian pacemaker neurons lengthened 24-hour periodicity in free-running locomotor behaviors. Taken together, our results provide new insights into the function of hnRNP K as a transcriptional amplifier of DBP, which acts rhythmically through its intracellular localization by the ERK phosphorylation and as an mRNA stabilizer along with its physiological significance in circadian rhythms of Drosophila.