uridine diphosphate galactose See uridine di- phosphate glucose. uridine diphosphate glucose (UDPG) a com- pound (shown below) that acts as a coenzyme and substrate to various enzymes. UDPG may be con- verted to uridine diphosphate galactose by the en- zyme epimerase.
These coenzymes play essential roles in carbohydrate metabolism. uridylic acid See nucleotide. Urkingdom a kingdom that some authorities have suggested be erected to house the Archaebacteria (q.v.). URL uniform resource locator (q.v.). Usn RNAs the U class of small nuclear RNAs (q.v.). These molecules range in size from 60 to 216 nucleotides and are rich in uridine. Five of Usn RNAs (U1, U2, U4, U5, and U6) have been studied in the most detail.
The first four all have a trimethyl- guanosine cap structure at the 5′ end. Most Usn RNAs are associated with seven proteins, some of which are common to all five RNAs, while others are specific for U1 or U2.
These snRNPs make up about a third of the mass of the spliceosomes, and they function in the excision and splicing reactions that take place in this organelle. See cajal body, exon, intron, lupus erythematosus, posttranscriptional modification, splice junctions.
Ustilago a genus in the Basidiomycota (see Appendix A). These are yeastlike smut fungi, and two species, U. maydis and U. violacea, have been subjects of genetic research, especially in terms of recombination-defective and radiation-sensitive mu- tations. uteroglobin a protein synthesized by the cells of the rabbit endometrium and present in the uterine fluids. Uteroglobin is composed of two identical sub- units of 70 amino acids, and the subunits are held together by two disulphide bridges. Reduced utero- globin binds steroids, such as progesterone.
There is one uteroglobin gene per genome. It is 3 kilobases long and contains two intervening sequences and three exons. Some authors have proposed that ute- roglobin exerts a stimulatory effect on the blastocyst (hence its other name, blastokinin). utrophin a gene which maps to 6q24 and encodes DRP, a dystrophin-related protein. DRP has an amino acid sequence with an 80% identity to dys- trophin (q.v.), and it is synthesized in fetal muscle cells.
Since the urotrophin protein functions like dystrophin in early development, upregulation of this gene has been suggested as a possible therapeu- tic approach for the treatment of Duchenne muscu- lar dystrophy (q.v.). UV ultraviolet radiation (q.v.). UV-induced dimers See thymine dimers. UV reactivation a phenomenon in which survival of an ultraviolet-irradiated lambda phage is greater on an irradiated host than on an unirradiated host. The repair mechanism involved in UV-reactivation utilizes an error-prone replication system of the host such as that of the SOS response (q.v.).
Uridine diphosphate glucose (UDPG)