FIGURE 1-46 Structures of retinol (vitamin A), 9-frans-retinoic acid, and 9-ris-retinoic acid.

their formation allows even more sophisticated regulation of gene transcription.

Considerable intellectual effort has been invested to evaluate the evolutionary origins and relationship of the nuclear receptor gene family and their possible relationships to other nuclear transcription factors. Through the use of protein sequence comparisons of the DNA-binding domain (=ยป75-77 amino acids long; see Figure 1-27), the current conclusion is that all of the DNA-binding domains evolved from a single common precursor that was the ancestor of arthropods and vertebrates. With respect to the two zinc fingers (see Figure 1-42), the prevailing view is that they evolved as single, separate structural units rather than by duplication of a single ancestral zinc finger sequence. Thus, it appears that the steroid nuclear receptors do not share along with other transcription factors, or other zinc finger proteins, a common ancestral protein.

Our understanding of small molecule (steroid hormone, retinoic acid, and thyroid hormone) receptor regulation of gene transcription is still emerging. It is to be anticipated that there will be significant advances in the future involving how nuclear transcription factors, hormone receptors, etc. function to achieve regulation of gene transcription.

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