Graduate School of Biomedical Sciences, Biochemistry and Molecular Pharmacology
Dissertations, UMMS; Caenorhabditis elegans Proteins; Carrier Proteins; 3' Untranslated Regions; RNA, Messenger
In most metazoans, early embryogenesis is controlled by the translational regulation of maternally supplied mRNA. Sequence-specific RNA-binding proteins play an important role in regulating early embryogenesis, yet their specificities and regulatory targets are largely unknown. To understand how these RNA-binding proteins select their targets, my research focused on the C. elegans CCCH-type tandem zinc finger protein POS-1. Embryos lacking maternally supplied POS-1 die prior to gastrulation, and exhibit defects in the specification of pharyngeal, intestinal, and germline precursor cells. To identify the regulatory targets that contribute to the POS-1 mutant phenotype, we set out to determine the sequence specificity of POS-1 in vitro, and then use this information to identify regulatory targets in vivo.
Using a candidate-based search, we identified a twelve-nucleotide fragment of the mex-3 3' untranslated region (3' UTR) to which POS-1 binds with high affinity. Using quantitative fluorescent electrophoretic mobility shift assays, I determined the affinity of the RNA-binding domain of POS-1 for a panel of single nucleotide mutations of this sequence, and then defined a consensus binding element based on this dataset. POS-1 recognizes the degenerate element UAU 2-3 RDN 1-3 G, where R is any purine (adenosine or guanine), and D is any base except cytosine. A bioinformatics analysis revealed the presence of this element in approximately 40% of C. elegans 3' UTRs, suggesting that POS-1 is capable of binding to and perhaps regulating many transcripts in vivo. POS-1 binding sites alone are not sufficient to pattern the expression of a reporter, suggesting that other factors may contribute to POS-1 specificity.
To address the mechanism of POS-1-mediated translational regulation, I investigated the translational regulation of the C. elegans Notch homolog glp-1. Previous work demonstrated that glp-1 translation is repressed in the early embryo in a POS-1-dependent fashion, though it was not clear if this regulation was direct. The glp-1 3' UTR contains two POS-1 binding sites within five nucleotides of each other, and these sites are within a thirty nucleotide region of the 3' UTR required for proper spatiotemporal translation of glp-1. The POS-1 sites overlap with a negative regulatory element that is recognized by GLD-1, and a positive regulatory element recognized by an unknown factor. Both POS-1 and GLD-1 bind to an RNA containing these sites in vitro, and POS-1 competes with GLD-1 for binding. Both proteins are required for translational repression of a glp-1 3' UTR reporter in embryos. Furthermore, only one of the two POS-1 binding sites is required for repression, and the required site is wholly contained within a previously characterized positive regulatory element. Based on this, we propose that POS-1 does not regulate its targets by recruiting regulatory machinery, but instead by competing with factors that do. Thus, sites of POS-1 regulation are highly context dependent, which may contribute to POS-1 specificity.
Farley, BM. Sequence and Target Specificity of the C. elegans Cell Fate Specification Factor POS-1: A Dissertation. (2012). University of Massachusetts Medical School. GSBS Dissertations and Theses. Paper 629. https://escholarship.umassmed.edu/gsbs_diss/629
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