UC San Diego

MicroRNAs (miRNAs) are small non-coding RNAs, ~22 nucleotides (nt) long, with major roles in gene regulation. MiRNAs bind imperfectly to complementary sequences in the 3’ untranslated region of target messenger RNAs (mRNAs) causing translational repression and destabilization. A single miRNA has the potential to regulate hundreds of different mRNA targets, highlighting the importance of miRNAs in almost all cellular pathways. Originally

discovered as part of the Caenorhabditis elegans (C. elegans) developmental timing pathway, miRNAs were soon found in a multitude of other organisms, including humans. MiRBase, an online database for miRNAs, now lists >35,000 miRNAs, in >200 different species, including viruses, though many of their roles remain to be characterized. Because misregulation is often associated with disease, especially cancer, exploring miRNA biogenesis is critical for understanding the intricacies of disease development. Furthermore, the conserved temporal expression of various miRNAs highlights the importance of these regulators in pluripotency and development. Understanding how miRNAs are produced and regulated has been a major topic of study over the past 15 years. While the basic mechanisms of miRNA biogenesis and function have been uncovered, how these processes are regulated remains an outstanding problem.

There are multiple instances of transcriptional and post-transcriptional regulation during miRNA biogenesis. In Chapter I, I introduce much of the latest understanding about the mechanisms of miRNA biogenesis and regulation. Details about the discovery of miRNAs, C. elegans as a model, as well as general information on biogenesis and targeting, can be found in Chapter II. I worked on several projects investigating post-transcriptional regulation of miRNA biogenesis in C. elegans. In Chapter III, I identify and characterize the primary lin-4 transcripts, and demonstrate how a conserved RNA binding protein, RBM-28, regulates mature lin-4 expression, but not primary or precursor. My investigation in Chapter IV led to notable insights on how splicing pri-let-7 leads to a secondary structure rearrangement that facilities recognition by the Microprocessor. My survey of polycistronic worm miRNAs discussed in Chapter V indicates that there are many more examples awaiting further study. Overall, this research describes novel examples of post-transcriptional regulation of miRNAs.