The Questions of Developmental Biology

The third functional region of MITF is its trans-activating domain. This domain includes
a long stretch of amino acids in the center of the protein. When the MITF dimer is bound to its
target element in a promoter or enhancer, the trans-activating region is able to bind a TAF called
p300/CBP. The p300/CPB protein is a histone acetyltransferase enzyme that can transfer acetyl
groups to each histone in the nucleosomes (Ogryzko et al. 1996; Price et al. 1998). This
acetylation of the nucleosomes destabilizes them and allows the genes for pigment-forming
enzymes to be expressed. Recent discoveries have shown that numerous transcription factors
operate by recruiting histone acetyltransferases. It is thought that once the nucleosomes are
destabilized, other transcription factors and RNA polymerase can bind more easily to the DNA in
that region.
The Pax6 transcription factor, which is needed for mammalian eye, nervous system, and
pancreas development, contains two potential DNA-binding domains. The major DNA-binding
site of the Pax6 protein resides at its amino-terminal end, and these amino acids interact with a
specific 20 26-base-pair sequence of DNA (Figure 5.10; Xu et al. 1995). Such Pax6-binding
sequences have been found in the enhancers of vertebrate lens crystallin genes and in the genes
expressed in the endocrine cells of the pancreas (insulin, glucagon, and somatostatin) (Cvekl and
Piatigorsky 1996; Andersen 1999). When Pax6 binds to a
particular site in an enhancer or promoter, it can either
activate or repress that gene. The trans-activating domain of
Pax6 is rich in proline, threonine, and serine. Mutations in
this region cause severe nervous system, pancreatic, and optic
abnormalities in humans Glaser et al. 1994.
The use of Pax6 by
different organs
demonstrates the modular
nature of transcriptional
regulatory units.
Figure 5.11 shows two gene regulatory regions that use Pax6. The first is that of the chick 1
lens crystallin gene. This gene has a promoter containing a site for TBP binding and an upstream
site that binds Sp1, a general transcriptional activator found in all cells. The gene also has an
enhancer in its third intron that controls the time and place of crystallin gene expression. This
enhancer has two Pax6-binding sites. The crystallin gene will not be expressed unless Pax6 is
present in the nucleus and bound to these enhancer sites. As mentioned in Chapter 4, Pax6 is
present during early development in the central nervous system and head surface ectoderm of the
chick. Moreover, this enhancer has a site for another transcription factor, the Sox2 protein. Sox2
is not usually found in the outer ectoderm, but it appears in those outer ectodermal cells that will
become lens by virtue of their being induced by the optic vesicle evaginating from the brain
(Kamachi et al. 1998). Thus, only those cells that contain both Sox2 and Pax6 can express the
lens crystallin gene. In addition, there is a third site that can bind either an activator (the EF3
protein) or a repressor (the EF1 protein) of transcription. It is thought that the repressor may be
critical in preventing crystallin expression in the nervous system. Thus, enhancers function in a
combinatorial manner, wherein several transcription factors work together to promote or inhibit
transcription.