Yeast artificial chromosomes (YACs) genetically
yeast artificial chromosomes (YACs) genetically engineered circular chromosomes that contain ele- ments from chromosomes contributed by Saccharo- myces and segments of foreign DNAs that can be much larger than those accepted by conventional cloning vectors (q.v.). As shown in the diagram on page 480, YACs are generated from synthetic mini- chromosomes that contain a yeast centromere (C), a replication orgin (RO), and fused telomeres (TR and Tr).
In addition, the circular chromosome con- tains three marker genes (M1, M2, and M3), which when expressed, allow selection of the cells carrying the plasmid and sites 1 and 2, which allow specific restriction endonucleases (q.v.), to break the mole- cule. Cleavage at 1 opens the ring, while cleavage at 2 generates centric and acentric fragments with ends that will accept foreign DNA fragments. Once these are ligated, an artificial chromosome is generated with a short and a long arm.
This contains the spliced segment of foreign DNA to be cloned. Such artificial chromosomes are distributed normally dur- ing subsequent yeast divisions, and so colonies con- taining YACs are generated. In cells possessing the insert, the M1 and M3 markers are expressed, but the damaged M2 is not. So religated YACs can be distinguished from unbroken plasmids. YACs can accept DNA inserts up to 1,000 kilobases long.
Compare with bacterial artificial chromosomes (BACs) and P1 artificial chromosomes (PACs). See Appendix C, 1987, Burke, Carle, and Olson; DNA vector, kilobase, plasmid cloning vectors. yeast nucleic acid See nucleic acid. yeast two-hybrid system an in vivo (q.v.) method for identifying protein-protein interactions, based on the properties of a transcriptional activator protein.
The simplest version of this system is based on the yeast protein, GAL4 (q.v.), whose DNA-binding do- main (BD) binds with an upstream activator se- quence (UAS) and the activation domain (AD) in- teracts with the transcription complex to stimulate transcription of a downstream gene (see illustration A). In this two-hybrid scheme, two plasmids (q.v.) encoding two hybrid proteins are constructed and introduced into yeast cells.
One hybrid contains the GAL4 BD fused to a known protein (protein X), and the second hybrid is a fusion between the GAL4 AD and a second protein (protein Y).
These hybrids are coexpressed in a yeast strain lacking GAL4 activ- ity and containing a reporter gene (q.v.), such as the bacterial lac Z gene, with a binding site for GAL4. Either hybrid by itself is incapable of inducing tran- scription (illustration B). An interaction between proteins X and Y, however, brings the BD and AD in close proximity, and GAL4 activity is reconsti-
tuted (illustration C). This leads to transcriptional activation of the reporter gene and allows X-Y inter- action to be monitored by β-galactosidase (q.v.) ac- tivity. This approach has been modified to screen protein sequences in libraries. In this case, the sec- ond hybrid is a fusion between the GAL4 AD and proteins encoded by genomic (q.v.) or cDNA library (q.v.) sequences. Interaction between the known protein and a protein encoded by one of the library plasmids is detected by expression of the reporter gene. See Appendix C, 1989, Field and Song; GAL4, lac operon.
yeast two-micron plasmid See FLP/FRT recombi- nation. Yersinia pestis a Gram-negative bacterium that is the cause of plague. The genus gets its name from Alexandre Yersin, a colleague of Pasteur, who iso- lated the bacterium in 1894.
Bubonic plague is maintained in rat populations and transmitted to hu- mans by the bites of rat fleas. Pneumonic plague oc- curs under crowded conditions where infected per- sons spread the bacteria in respiratory droplets which are directly inhaled by nearby people. Y. pestis appears to have been a relatively harmless gut pathogen until about 1,500 years ago when it picked up genes that allowed it to colonize fleas and to mul- tiply in the bloodstream of humans.
Its genome has been sequenced and found to consist of a 4.65 Mb chromosome and three plasmids of 96.2 kb, 70.3 kb, and 9.6 kb. See Appendix A, Bacteria, Proteobacteria. Y fork the point at which a DNA molecule is being replicated; the two template strands of the parental molecule separate, forming the arms of a Y- shaped structure. The unreplicated double-stranded DNA distal to the arms forms the base of the Y. See replication of DNA.
Y linkage genes located on the Y sex chromosome, exhibiting holandric (q.v.) inheritance. See Oryzias latipes. yolk the complex collection of macromolecules and smaller nutrient molecules with which the oo- cyte is preloaded prior to fertilization. See lipovitel- lin, phosvitin, vitellogenin. Y-suppressed lethal a sex-linked, recessive lethal that causes death of XO Drosophila melanogaster but allows survival of normal males.
Z atomic number, the number of protons in the nucleus of the neutral atom. Z chromosome the sex chromosome found in both heterogametic females and homogametic males. See W, Z chromosomes. Z DNA See deoxyribonucleic acid. Zea mays spp mays maize or Indian corn, one of the world’s most important food sources. The hap- loid chromosome number is 10, and cytological maps are available for the pachytene chromosomes.
The genome contains 2.4 gbp of DNA, of which half is located in retrotransposons (q.v.).
See Appendix A, Plantae, Angiospermae, Monocotyledoneae, Grami- nales; Appendix C, 1909, Shull; 1913, East and Em- erson; 1928, Stadler; 1931, 1933, 1934, 1938, Mc- Clintock; 1938, Rhoades; 1950, McClintock; 1964, Mertz et al.; 1984, Pohlman et al.; corn, kwashiorkor, opaque-2, R genes of maize, teosinte. zeatin See cytokinins. zebra fish a popular, easy-to-rear aquarium fish. Its scientific name is Danio rerio (q.v.).
In the earlier literature its genus is sometimes given as Branchy- danio. zebras See Equidae.
Zebu the Brahman (q.v.) breed of cattle. zein a group of alcohol soluble proteins that func- tion as storage proteins in maize kernels. The pro- teins are encoded by a multigene family, are synthe- sized in the developing endosperm, and account for more than 50% of the protein in mature seeds.
Un- fortunately zein is practically devoid of lysine. See kwashiorkor, opaque-2, Zea mays. zero-order kinetics the progression of an enzy- matic reaction in which the formation of product proceeds at a linear rate with the time. This rate is not increased if additional substrate is added. See first-order kinetics. zero sum assumption an aspect of the Red Queen hypothesis (q.v.) proposing that the beneficial effect enjoyed by a species in evolutionary advance is pre- cisely matched by the sum of the negative effects experienced by all other species in the community.
The zero sum assumption derives in part from the notion that the total resources available in the sys- tem is constant and the rate of evolution will also be constant. zero time binding DNA strands of DNA contain- ing intramolecular repeats that form duplexes at the start of a reassociation reaction. Zimmermann cell fusion a technique developed by Ulrich Zimmermann in which cells are exposed to a low-level, high-frequency electric field that ori- ents them into chains. A direct current pulse is then used to open micropores in adjoining cell mem- branes.
These micropores allow mixing of the cyto- plasms, and the cells may eventually fuse. The Zim- mermann technique may also alter the permeability of the plasmalemma so that DNA fragments the size of genes can enter the cell. zinc a biological trace element. Atomic number 30: atomic weight 65.37; valence 2+; most abundant isotope 64Zn, radioisotope 65Zn, half-life 250 days, radiation emitted—positrons. Zn zinc. zinc finger proteins proteins possessing tandemly repeating segments that bind zinc atoms. Each seg- ment contains two closely spaced cysteine molecules followed by two histidines.
Each segment folds upon itself to form a fingerlike projection.
The zinc atom is linked to the cysteines and histidines at the base of each loop as shown here, where C circles represent cysteine molecules, H circles represent histidine molecules, and unlabeled circles represent the other amino acids of the polypeptide finger.
The zinc fin- gers serve in some way to enable the proteins to bind to DNA molecules, where they regulate transcrip- tion. See Appendix C, 1985, Miller et al.; 1987, Page et al.; androgen receptor (AR), motifs, transcription factors, vitamin D receptor (VDR), Wilms tumor.