| The let-7 family of microRNAs (miRNAs) are important regulators of developmental timing and cellular differentiation. As a member of the C. elegans heterochronic pathway, let-7 controls cell fate transitions from larval stage 4 (L4) to adulthood by post-transcriptionally down-regulating lineage-abnormal 41 (lin-41) and hunchback-like 1 (hbl-1). Primary let-7 (pri-let-7) transcripts are up-regulated in the L3, yet little is known about what controls this transcriptional up-regulation. I sought factors that either turn on let-7 transcription or keep it repressed until the correct developmental time.;Here I report that one of let-7's targets, the transcription factor hbl-1, is responsible for inhibiting the transcription of let-7 in specific tissues until the L3 stage. hbl-1 is a known developmental timing regulator and inhibits adult development in larval stages. Therefore, one of hbl-1's important functions is to maintain larval stage fates by inhibiting let-7 expression. Additionally, my results reveal let-7 as the first known target of hbl-1 in C. elegans and suggest a negative feedback loop mechanism for let-7 and hbl-1 in regulating cell differentiation.;In this work, I further characterize a temporal regulatory element (TRE) in the let-7 promoter and an unknown transcriptional activator designated the temporal regulatory element binding protein (TREB) that binds this DNA sequence. I show that point mutating specific bases in the inverted repeat region of TRE abrogates TREB binding. I contribute to the knowledge of TREB's physical characteristics, including an estimated molecular weight and an isoelectric point. Lastly, I present the examination of candidate TREB proteins and their genetic interactions with let-7.;The high conservation of let-7 and other members of the C. elegans heterochronic pathway implies that the information presented here may be useful in understanding the transcriptional regulation of let-7 in higher organisms. As let-7s critical role in development and cellular differentiation has implicated its mis-expression as a factor that contributes to many diseases, understanding what regulates its expression could translate into diagnostic tools and treatments for these disease states. |
