The second category of HMG-class proteins consists of those with a single HMG box and that bind DNA in a highly sequence-specific manner. This group includes the HMG-1 protein, ubiquitous binding factor (UBF), and mitochondrial transcription factor 1 (MT-TF1). One group is characterized by proteins containing multiple HMG boxes, having a general affinity for binding DNA independent of its sequence.
Two basic types of HMG-class proteins can be delineated. The SOX genes belong to a large group of genes in which the DNA-binding domain is called a high mobility group (HMG) box ( 12). While SOX genes have not yet been as well characterized, they appear to also play important roles in early development and to have interesting associations to human disease. Mutations in other PAX genes also cause human diseases, including coloboma and renal anomalies (PAX2), and Waardenburg syndrome (PAX3) ( 10, 11). Interestingly, mutations in this gene cause either aniridia or autosomal dominant keratitis, two congenital eye defects in humans ( 7– 9). Within the PAX ( paired bo x) family of genes ( 6), PAX6 is a master eye development control gene in organisms as diverse as insects, cephalopods, and humans.
An example is the HOM/HOX ( homeob ox-containing) gene complexes which play a role in early pattern formation in the Drosophila larva and have also been implicated in pattern formation in early vertebrate development ( 5). Many of these genes were first studied in Drosophila. These gene families encode proteins characterized by well-defined DNA-binding motifs, such as zinc-fingers or homeobox domains, typically highly conserved across species ( 3, 4). Large “families” of transcription factors which are involved in the control of developmental pathways have been identified.
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