Exonucleases

E xp res s i v i t y 675 sequential assembly of subunits leading to the dual incision event. These six factors are XPA, RPA, TFIIH (XPB and XPD plus four additional polypeptides), XPC.HR23B, XPG, and XPF.ERCC1. Following damage recognition by XPA and RPA, XPC and TFIIH are recruited to the damage site (see Figure 1) to form the first stable preincision complex. The initial, localized helical denaturation resulting from DNA damage is extended both 50 and 30 by the helicase activities of two TFIH subunits, XPB and XPD. XPC helps to stabilize this open complex and, furthermore, XPC is a molecular matchmaker that dissociates after recruiting and positioning XPG 30 to the DNA damage. The last factor to assemble is the XPF.ERCC1 heterodimer. Dual incisions follow rapidly with XPG nicking the DNA at the sixth  3 phosphodiester bond 30 to the damage and XPF. ERCC1 hydrolyzing at the 20th  5 bond 50 to the lesion. The 24±32 nucleotide-long oligomer containing the damaged base is released from the DNA (excision) and repair factors rapidly dissociate following the dual incision event leaving a gapped substrate. In subsequent steps, DNA polymerases d and e and their accessory factors, PCNA and RFC, assemble at the gapped molecule and the undamaged strand is used as a template for precise resynthesis of the DNA. The repair patch size matches the size of the excision gap and, when the gap is filled to the 30 end, the repair patch is ligated to the parental DNA by a ligase.

Further Reading

Petit C and Sancar A (1999) Nucleotide excision repair from E. coli to man. Biochimie 81: 15±25. Sancar A (1996) DNA excision repair. Annual Review of Biochemistry 65: 43±81. Wood R D (1996) Excision repair in eukaryotes. Annual Review of Biochemistry 65: 135±167.

See also: DNA Repair; Xeroderma Pigmentosum

Exon See: Introns and Exons

Exonucleases Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.1836

Exonucleases are enzymes that digest the ends of a piece of DNA. The nature of the digestion is usually specific (e.g. 50 or 30 exonuclease). Exonuclease III

(exo III), for example, is used to prepare deletions in cloned DNA, or for DNA footprinting. See also: Endonucleases; Footprinting; Nuclease

Expression Vector Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.1837

An expression vector is a vector designed for the expression of inserted DNA sequences propagated in a suitable host cell. The inserted DNA is transcribed and translated by the host's cellular machinery. See also: Vectors

Expressivity J A Fossella Copyright ß 2001 Academic Press doi: 10.1006/rwgn.2001.0440

Expressivity refers to the variation seen among individuals expressing a particular trait or mutant phenotype. `Variable expressivity' is the term used to describe a trait or mutant phenotype that fluctuates in degree or severity from individual to individual in a population. For example, all individuals of a population expressing a trait or mutant phenotype such as `spotted' may show an identical number of spots. This would be an example of low or nonvariable expressivity. Alternatively, some individuals may have many spots while others only a few and many with an intermediate number of spots. This would be an example of variable expressivity, since all the individuals express the trait or mutant phenotype of `spotted' but vary in the degree of spotting. Expressivity is similar in meaning to `penetrance' and the two terms are often used together when describing mutations. For example, certain weak alleles of the W locus seen in mice result in white coat color spots. These mutant alleles are said to show reduced penetrance and variable expressivity. The distinction between penetrance and expressivity is that penetrance refers to the genotype while expressivity refers to the phenotype. In this example, only some of the mice that carry the W /‡ genotype show any spots at all. This is an example of reduced penetrance. Of the animals that show the spotted `phenotype' however, some tend to show much spotting