Universal tree of life
unwinding proteins proteins that bind to, destabi- lize, and unwind the DNA helix ahead of the repli- cating fork.
See Appendix C, 1970, Alberts and Frey; gene 32 protein. u orientation See n orientation. up promoter mutations mutations in promoter sites that increase the rate of initiation of transcrip- tion; promoters with this property are called “high level or strong promoters.” upstream See strand terminologies, transcription unit. Compare with downstream. uracil See bases of nucleic acids. uracil fragments during DNA replication in E. coli, polymerases I and III occasionally make mis- takes and incorporate dUTP instead of TTP.
Several enzymes remove these uracils from both leading and lagging strands, creating “uracil fragments.” urea See ornithine cycle. urease a nickel-dependent metaloenzyme that cat- alyzes the hydrolysis of urea to form ammonia and carbon dioxide.
Urease was the first enzyme to be isolated in pure, crystalline form. The source was seeds from the jack bean plant (Canavlia ensiformis). See Appendix C, 1926, Summer. urethane a carcinogen that induces tumorous nod- ules in the lungs of mammals.
URF unidentified reading frame. The open reading frames (ORFs) uncovered in the DNA of a species of interest are matched against those in large data- bases that contain all known ORFs from all kinds of organisms.
A substantial number of the new ORFs are left over that do not resemble genes that encode any known proteins.
These ORFs are therefore also URFs. Such genes are sometimes called orphans. (q.v.). uric acid the end product of nucleic acid catabo- lism in mammals; the main nitrogenous constituent of the urine of reptiles and birds.
See structure on page 467. uridine See nucleoside.