A hallmark of circadian clock components is their rhythmic expression profile. Nocturnin, a rhythmically expressed gene, was first identified in the Xenopus laevis retina. Since then, nocturnin homologues have been identified in several other organisms, including mouse, where it is also expressed with a high amplitude circadian rhythmicity. Nocturnin is similar in sequence with CCR4, a protein recently identified as a cytoplasmic deadenylase involved in mRNA turnover. In this work, the role of nocturnin in both the circadian clock and mRNA stability was investigated. Investigation of the nocturnin protein expression profile showed that nocturnin is located primarily in the cytoplasm in heterologous cells and in the Xenopus laevis retina, and similar to the mRNA expressin profile, nocturnin protein is rhythmically expressed in the retinal photoreceptor cells. Sequence analysis of nocturnin revealed that it belongs to a family of magnesium-dependent nucleases including phosphatases and proteins involved in DNA repair. Although nocturnin protein tested negative for phosphate removal and DNA repair function, the discovery that CCR4 possessed deadenylase activity prompted testing of nocturnin for this activity. When incubated with a mature mRNA, nocturnin cleaves the poly(A) tail in a magnesium-dependent manner. Further characterization of this deadenylase activity revealed that nocturnin is a poly(A)-specific exonuclease. Comparing the rhythmic expression of nocturnin to the constitutive expression of another deadenylase, PARN, in the Xenopus retina suggested that nocturnin may deadenylate circadian-related messages. To further investigate the deadenylase function of nocturnin, I performed an immunoprecipitation screen to identify binding partners of the mouse homologue of nocturnin. The association of nocturnin and Lrpl30, an RNA-binding protein implicated in mRNA stability, is characterized in this work. Nocturnin probably associates with Lrpl30 through its leucine zipper-like motif, and the interaction of these two proteins is not dependent on the presence of RNA. The role of Lrp 130 in nocturnin deadenylation is unknown, although given its potential role in mRNA stability it is possible that Lrp 130 inhibits nocturnin function. Future work to characterize other proteins that associate with nocturnin and investigate the role of the nocturnin in circadian gene expression is important for our understanding of this protein. Note: Abstract extracted from PDF file via OCR.