Chapter 18: Regulation of Gene Expression in Eukaryotes

Regulation of Gene Expression in Eukaryotes details the intricate control of transcription initiation, where general transcription factors bind to core promoters (such as the TATA box) to establish basal transcription, while specific activator and repressor proteins interact with distant enhancers and promoter-proximal elements to modulate gene activity. The text explores critical protein structural motifs, including helix-turn-helix, zinc finger, and leucine zipper domains, and explains how activators recruit coactivators like the Mediator complex to facilitate RNA polymerase II function. Significant attention is given to chromatin remodeling, describing how histone acetyltransferases (HATs/KATs) and ATP-dependent complexes like SWI/SNF alter nucleosome positioning to expose DNA, whereas histone deacetylases (HDACs) and DNA methylation in CpG islands contribute to gene silencing and heterochromatin formation. The chapter illustrates these concepts with specific regulatory models, such as the yeast GAL system involving Gal4p and glucose repression, and steroid hormone signaling pathways where hormone-receptor complexes bind directly to DNA response elements like GREs. Combinatorial gene regulation is exemplified by the developmental control of the Drosophila even-skipped gene, driven by gradients of maternal-effect and segmentation proteins like Bicoid and Krüppel. Epigenetic phenomena are thoroughly examined, particularly genomic imprinting at the Igf2/H19 locus, which relies on methylation-sensitive insulators and CTCF binding to ensure parent-specific expression. The summary also covers post-transcriptional regulation, including alternative splicing and polyadenylation that generate protein isoforms like calcitonin and CGRP, as well as RNA interference (RNAi) pathways where microRNAs (miRNAs) and short interfering RNAs (siRNAs) direct gene silencing through the RISC complex. Finally, the text discusses translational control mechanisms and protein degradation pathways mediated by ubiquitin tags and the proteasome.