Molecular Genetics of Steroid and Thyroid Hormone Receptors 
A point of potential physiological significance 
is that 9-cis RA also binds to and transactivates 
both RXRs and RARs and may thus serve as a com- 
mon or "bifunctional" ligand. Conversion of the 
all-trans to the 9-cis isomer could provide a 
novel means for differential cell-speciftc regula- 
tion of the activity of these retinoid pathways. 
The hypothesis that 9-cis RA may be functionally 
distinct from its all-trans precursor raises the in- 
triguing possibility that the regulation of its iso- 
merization could be a key step in retinoid physiol- 
ogy. It is unknown whether this reaction is 
catalyzed by an enzyme. 
The 3-4-5 Rule 
Members of the receptor superfamily modulate 
target gene expression by binding as either homo- 
or heterodimers to hormone response elements 
(HREs). We recently described the properties of 
direct repeats of the consensus half-site sequence 
AGGTCA as HREs for nuclear receptors. Receptor 
specificity for binding and activation was shown 
to be conferred through the number of nucleo- 
tides separating the two half-sites. Spacers of 3, 4, 
or 5 nucleotides were originally shown to serve 
as optimal response elements for the vitamin D 
receptor (VDR), thyroid hormone receptor (TR), 
and RAR, respectively. We now refer to this physi- 
ological code built into HREs as the "3-4-5 rule." 
More recently, we have characterized in the up- 
stream regulatory region of the cellular retinol- 
binding protein type II (CRBPII) gene an HRE 
that confers selective responsiveness to the RXRs. 
This response element consists of tandem repeats 
of the AGGTCA sites separated by a single 
nucleotide. 
The ability of the VDR, TR, and RAR to recog- 
nize their cognate response elements is depen- 
dent upon their ability to form a heterodimeric 
complex with an unknown nuclear factor. Re- 
markably, the search for this factor has recently 
identified RXR as a common heterodimeric 
partner for the VDR, TR, and RAR. These results 
emerged from our initial finding of a functional 
interaction between the RAR and RXR, followed 
by the demonstration that these two receptors 
form stable heterodimers in solution. Further- 
more, the heterodimer binds target DNA with 
more than 100-fold increased efficiency over ei- 
ther partner alone. 
Similarly, RXR heterodimers with the VDR and 
TR have greatly increased affinity for the respec- 
tive cognate response elements. Apparently RXR 
is serving as a type of master receptor, gating the 
activities of vitamin D, thyroid hormone, and reti- 
noic acid. As noted above, however, RXR can act 
independently of all these receptors when re- 
sponding to its novel hormone 9-cis RA. Despite 
the apparent level of complexity revealed by 
these interactions, it appears that the physiologi- 
cal response is built upon a series of simple prin- 
ciples in which heterodimers, each consisting of 
an RXR component, interact with common DNA 
sequences, each varying by a single nucleotide. It 
is through this simplicity that such exquisite 
specificity can be maintained and thus permit co- 
ordinate control of a vast gene network in a 
highly selective and orderly fashion. 
Opposite: Expression pattern of the genes for RXRa, ^, and 7 in a mouse embryo. RXRs are 
receptor proteins that mediate the actions of the newly described hormone, 9-cys retinoic acid. 
Parasagittal sections from the embryo taken at gestation day 16.5 were hybridized in situ with 
radioactive probes that mark the specific areas where RXR mRNA is expressed. After exposure to 
x-ray film, the resulting image is digitally scanned into a computer and colorized. Dark red to 
yellow colors represent low to high levels of RXR expression. Thus the figure demonstrates that 
RXRa is strongly expressed in skin and metabolic organs such as intestine; RXR^ is expressed 
ubiquitously; RXRy shows marked expression in the pituitary and corpus striatum, the brain 
center that regulates muscle coordination and is affected in parkinsonian disorders. 
Reprinted with permission from Mangelsdorf D.J., Borgmeyer, U., Heyman, R.A., Zhou,J.Y., 
Ong, E.S., Oro, A.E., Kakizuka, A., and Evans, R.M. 1992. Genes Dev 6:329-344. Computer 
photograph by Jamie Simon, the Salk Institute for Biological Studies. 
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