Molecular Biology Terms Starting With Z

Z-Form DNA is a left-handed double-helical conformation of DNA with a zigzag phosphodiester backbone, favored by alternating purine-pyrimidine sequences under conditions of high salt concentration or negative superhelical stress.

Alexander Rich and colleagues discovered Z-DNA in 1979 by solving the crystal structure of the synthetic hexanucleotide d(CGCGCG), which unexpectedly adopted a left-handed helix rather than the familiar right-handed B-form. The backbone zigzags because alternating syn and anti glycosidic bond conformations place successive nucleotides in different orientations, producing a single deep groove instead of the major and minor grooves of B-DNA. Z-DNA forms transiently behind a transcribing RNA polymerase, where the torsional stress of unwinding the template generates negative supercoiling that stabilizes the left-handed conformation.

Specific Z-DNA binding proteins, including the ADAR1 editing enzyme and the innate immune sensor ZBP1, recognize this conformation through dedicated Z-alpha domains and link Z-DNA formation to antiviral defense and RNA editing.

ZAP-70 is a cytoplasmic tyrosine kinase of 70 kilodaltons that initiates intracellular signaling downstream of the T-cell receptor upon antigen recognition.

ZAP-70 contains two tandem SH2 domains that bind with high affinity to phosphorylated immunoreceptor tyrosine-based activation motifs on the CD3 zeta chain after T-cell receptor engagement. Once recruited to the receptor complex, ZAP-70 is phosphorylated by the Src-family kinase Lck at tyrosines 319 and 493, producing full enzymatic activation. Activated ZAP-70 then phosphorylates the adapter proteins LAT and SLP-76, which nucleate signaling complexes containing more than 20 proteins and propagate signals leading to T-cell proliferation and cytokine production.

Genetic deficiency of ZAP-70 in humans causes a form of severe combined immunodeficiency characterized by absent CD8-positive T cells and non-functional CD4-positive T cells, demonstrating its indispensable position in T-cell development.

Zinc Finger Nuclease is an engineered restriction enzyme that combines programmable zinc finger DNA-binding domains with the FokI nuclease to create targeted double-strand breaks at specific genomic sequences for genome editing.

Each zinc finger nuclease monomer contains three to six zinc finger motifs, with each finger recognizing a specific DNA triplet, linked to the FokI cleavage domain, which cuts DNA only when two FokI monomers dimerize. Two ZFN monomers must therefore bind opposite strands of the target DNA in the correct orientation and spacing, typically 5 to 7 base pairs apart, before cleavage occurs. This architecture provides recognition sites of 18 to 36 base pairs, substantially reducing off-target cutting compared with earlier non-specific nucleases.

Clinical applications include disrupting the CCR5 co-receptor gene in patient-derived T cells to confer resistance to HIV-1, a strategy tested in clinical trials beginning around 2009. Engineering functional ZFNs is technically demanding because the DNA-binding specificity of each zinc finger depends partly on its neighbors, a context-dependence that requires iterative selection or computational design to overcome.

Zinc Finger Protein is a protein containing one or more compact structural domains in which a zinc ion coordinates cysteine and histidine residues to stabilize a finger-shaped fold that mediates binding to DNA, RNA, or other proteins.

C2H2 zinc fingers, the most abundant type in the human proteome, each fold around a single zinc ion coordinated by two cysteines and two histidines, projecting an alpha-helical recognition helix into the major groove of DNA where specific amino acid side chains contact three to four base pairs. Different arrangements of zinc finger repeats create diverse binding specificities; a protein carrying multiple fingers in tandem can recognize extended DNA sequences with high selectivity. The transcription factor Sp1 uses three C2H2 zinc fingers to bind GC-rich promoter elements and regulate hundreds of human genes, including those controlling cell growth and differentiation.

Mutations in zinc finger proteins underlie several human diseases, including Wilms tumor, caused by mutations in the WT1 zinc finger transcription factor, and certain forms of limb malformation linked to disrupted ZRS enhancer-binding proteins.