uidA
The gene encoding Beta-glucuronidase (abbreviated GUS). Commonly used for constructing operon or gene fusions in organisms with high endogenous Beta-galactosidase activity. A variety of analogs exist that make it easy to detect GUS expression on solid medium and to assay the activity of the enzyme in cells.

Ultraviolet (UV) radiation
The part of the invivible electromagnetic spectrum (just below violet) with wavelengths between about 100-400 nm.

Undefined medium
A growth medium in which not all the components have been identified. Most "rich" media have a variety of complex, undefined componants.

Unequal cross-over
A recombination event that occurs between DNA molecules that are not fully aligned. For example, a crossover may occur between repeated DNA sequences resulting in the deletion or duplication the intervening DNA sequence.

Umber
The codon UGA. An less commonly used term for an opal codon.

Unidirectional replication
DNA replication that proceeds in only one direction along the DNA template.

Unselected marker
A genetic marker that may be co-inherited with an adjacent, selected marker but inheritance of the unselected marker is not demanded. For example, in a genetic cross you may select for one gene and screen for inheritance of an adjacent, unselected gene.

Up promoter mutation
A mutation that increasses expression from a promoter.

Upstream
A sequence located in front of a particular site relative to the direction of transcription and translation (i.e. located in the 5' direction relative to a particular site). For example, the lac promoter is located upstream of the lacZ structural gene.

Upstream activator sequence (UAS)
A DNA sequence some distance in front of a gene that increases transcription of the gene. Such sequences often stimulate binding of RNA polymerase to the promoter.

Uracil-N-glycosylase
A repair enzyme that removes uracil from DNA by cleaving the bond between the base and the sugar backbone. This enzyme ensures that DNA contains T instead of U residues.

UV absorbance spectroscopy
A method for measuring the concentration of a compound by determining the amount of ultraviolet radiation absorbed by a sample.

UV reactivation
A phenomenon in which survival of an ultraviolet-irradiated phage is greater upon infecting a host that has also been irradiated with UV than upon infecting a host that has not been irradiated with UV. The increased survival in the UV irradiated host is due to the induction of the SOS-repair system in the host.

UvrABC
An enzyme complex that functions as an endonuclease, cutting the DNA on both sides of DNA lesions that distort the double-helix (for example, T-T dimers). The DNA fragment is subsequently excised allowing resynthesis and repairing the DNA damage.