Different period gene repeats take 'turns' at fine-tuning the circadian clock

The repetitive region of the circadian clock gene period in Drosophila pseu doobscura consists predominantly of a pentapeptide sequence whose consensus is NSGAD. In D. melanogaster, this region is replaced by a dipeptide Thr-G ly repeat, which plays a role in the thermal stability of the circadian phe notype. The Thr-Gly repeat has been shown to form a type II or III beta-tur n, whose conformational monomer is (Thr-Gly)(3). Here we report, using conf ormational analyses, that both an NSGAD pentapeptide, and a polymer of the same sequence, form type II beta-turns. Thus two peptide sequences, whose a mino-acid composition is very different, nevertheless form the same seconda ry structure. The implications of these structures for clock function are d iscussed.