Molecular Biology Terms Starting With E

E3 Ligase is an enzyme that transfers ubiquitin from an E2 ubiquitin-conjugating enzyme to a specific target protein, determining which proteins in the cell receive ubiquitin modifications and what biological consequence those modifications produce.

E3 ubiquitin ligases fall into two main mechanistic classes based on their catalytic domains. RING domain E3 ligases bring the E2-ubiquitin thioester and the substrate into proximity, stimulating direct transfer of ubiquitin to a lysine residue on the target, while HECT domain E3 ligases instead accept ubiquitin onto their own active-site cysteine before transferring it to the substrate. The human genome encodes approximately 600 to 1,000 distinct E3 ligases, each recognizing substrates through dedicated receptor domains such as F-boxes in SCF complexes or SOCS boxes in Cullin-RING ligases, giving the ubiquitin system the breadth to regulate thousands of proteins.

Ubiquitin chains linked through lysine-48 of ubiquitin typically direct substrates to the 26S proteasome for degradation, while chains linked through lysine-63 more often alter protein localization or recruit DNA repair factors. The tumor suppressor p53 is ubiquitinated by at least three distinct E3 ligases, including MDM2, COP1, and CHIP, each responding to different cellular stress signals to tune p53 abundance and activity.

Elongation is the phase of transcription or translation during which the RNA or polypeptide chain grows by sequential addition of nucleotides or amino acids to the 3-prime end of the nascent RNA or the C-terminus of the nascent polypeptide.

During transcription elongation, RNA polymerase translocates along the DNA template strand in the 3-prime to 5-prime direction, reading each base and adding the complementary ribonucleotide to the 3-prime end of the growing RNA at rates of approximately 20 to 80 nucleotides per second in bacteria and 20 to 40 nucleotides per second in human cells. In translation elongation, the ribosome cycles through three steps: aminoacyl-tRNA delivery to the A site, peptide bond formation catalyzed by the peptidyl transferase center of the large subunit, and translocation of the ribosome by one codon in the 5-prime to 3-prime direction. Elongation factors accelerate and coordinate these steps; in bacteria, EF-Tu delivers aminoacyl-tRNA to the ribosome in a GTP-dependent manner, and EF-G drives translocation using GTP hydrolysis.

Bacterial ribosomes incorporate approximately 15 to 20 amino acids per second, while eukaryotic ribosomes proceed at roughly 3 to 6 amino acids per second, a difference that reflects structural and regulatory distinctions between the two systems.

Enhancer is a cis-acting DNA regulatory element that increases transcription of a target gene when bound by activator transcription factors, independent of its orientation relative to the promoter and capable of acting over distances of hundreds of thousands of base pairs.

Enhancers contact target gene promoters through chromatin looping, bringing bound activator proteins into proximity with the transcription machinery assembled at the promoter. Active enhancers are marked by specific histone modifications, particularly monomethylation of histone H3 at lysine 4 (H3K4me1) and acetylation of histone H3 at lysine 27 (H3K27ac), as well as open chromatin detectable by ATAC-seq or DNase I hypersensitivity assays. Many enhancers are themselves transcribed into short, unstable non-coding RNAs called enhancer RNAs (eRNAs), whose production correlates with enhancer activity, though whether eRNAs are mechanistically required or simply a byproduct of transcription factor binding remains debated.

Clusters of enhancers called super-enhancers, spanning 10 to 50 kilobases, drive exceptionally high expression of master regulator genes that define cell identity, and super-enhancers are disproportionately disrupted in cancer cells. Single nucleotide variants within enhancers account for a large fraction of the genetic variants associated with human disease in genome-wide association studies, underscoring that most disease-relevant variation falls outside protein-coding sequences.

Epigenome is the complete set of chemical modifications to DNA and histone proteins across all genomic loci in a given cell, which collectively regulate gene activity without altering the underlying DNA sequence.

The epigenome encompasses DNA methylation at cytosine residues within CpG dinucleotides, histone tail modifications including acetylation, methylation, phosphorylation, and ubiquitination at specific residues, chromatin accessibility profiles, and three-dimensional genome organization into compartments and topologically associating domains. Each cell type in a multicellular organism maintains a distinctive epigenome that determines which genes are transcriptionally active, poised for rapid induction, or stably silenced, establishing and preserving cell identity from a shared DNA sequence. Environmental exposures such as tobacco smoke, diet, and psychological stress alter epigenomic patterns, and aging is accompanied by progressive, reproducible changes in DNA methylation that are so consistent they form the basis of epigenetic clocks used to estimate biological age.

Many cancers acquire widespread epigenomic reprogramming, including hypermethylation of tumor suppressor gene promoters and global loss of DNA methylation, that drives tumor progression independently of DNA mutation.

Exonuclease is an enzyme that degrades nucleic acids by cleaving nucleotides sequentially from the 5-prime or 3-prime end of a DNA or RNA strand, producing mononucleotides or short oligonucleotides.

5-prime exonucleases including FEN1 and Exo1 remove approximately 1,000 to 30,000 nucleotides per minute and excise RNA primers synthesized by primase during DNA replication. DNA polymerase III incorporates a wrong base once every 10,000,000 nucleotides during synthesis, but its associated 3-prime to 5-prime exonuclease activity removes roughly 99.9% of these errors through immediate excision before extension resumes. RNase H1 degrades RNA in RNA-DNA hybrids by hydrolyzing phosphodiester bonds from either end, enabling removal of Okazaki fragment primers on the lagging strand.

At millisecond timescales during DNA repair, exonuclease activity processes damaged termini before recombinases like RAD51 engage. Mutations in exonuclease genes cause increased mutation rates, genomic instability, and elevated cancer risk in diseases including Werner syndrome and ataxia-telangiectasia.