MyoD: A Master Regulatory Gene for Myogenesis 
Harold M. Weintraub, M.D., Ph.D. — Investigator 
Dr. Weintraub is also a Full Member in the Division of Basic Sciences at the Fred Hutchinson Cancer Re- 
search Center and Affiliate Professor in the Department of Zoology at the University of Washington. He 
received his M.D.Ph.D. degree from the University of Pennsylvania School of Medicine and completed his 
postdoctoral studies at the Medical Research Council Laboratory of Molecular Biology in Cambridge, 
England. Prior to joining the staff at the Hutchinson Center, Dr. Weintraub was a member of the Depart 
ment of Biochemical Sciences at Princeton. He is a member of the National Academy of Sciences and the 
American Academy of Arts and Science. 
THE nucleus of a blood or skin cell has the 
capacity, when put back into an enucleated 
egg, to give rise to an entire animal, containing 
perhaps a thousand cell types. This demonstrates 
that a given cell type contains all the genetic in- 
formation needed to generate all other types. 
How, then, are the different cells in the body gen- 
erated? The answer is that different sets of genes 
are used to define different cell types — e.g., 
muscle-specific genes are expressed only in mus- 
cle cells, and nerve-specific genes, only in nerve 
cells. This solution has been confirmed in 
hundreds of experiments by many investigators 
over the past 10-20 years. 
But how are these different batteries of cell 
type-specific genes activated in the correct 
places and times in the embryo? The goal of our 
laboratory is to answer this question, and we have 
chosen to ask specifically how muscle cells are 
generated during development. 
MyoD is a Master Regulatory Gene 
for Myogenesis 
Our initial experiments identified a gene, 
MyoD, by the criteria that it was expressed in 
myoblasts but not in fibroblasts, nor in mutant 
muscle cell lines that had lost the capacity to dif- 
ferentiate into muscle. Detailed analysis showed 
that MyoD is expressed only in skeletal muscle 
cells and their committed myoblast precursors. 
When MyoD, however, is introduced into fibro- 
blasts using a viral long terminal repeat (LTR) to 
promote constitutive expression, these cells acti- 
vate the myogenic program and become muscle. 
A large number of other differentiated cell types 
— fat, pigment, brain, cartilage, etc. — can be 
converted to muscle by LTR-induced expression 
of MyoD. In the process, the normal program of 
these cells is usually turned off. 
We consider MyoD a "master regulatory gene" 
for myogenesis and view it as a "nodal point" in 
the flow of myogenic information. Many tem- 
poral and spatial cues from the early embryo im- 
pinge on MyoD to turn it on or off, but once acti- 
vated the gene can keep itself on (since it 
promotes its own transcription) and then turn on 
(directly or indirectly) the entire battery, per- 
haps hundreds, of muscle-specific genes. 
MyoD Protein Is a Transcription Factor That 
Activates Muscle-Specific Genes 
MyoD is a nuclear protein, 318 amino acids in 
length, that binds to muscle-specific enhancers. 
Many, if not most, muscle enhancers contain mul- 
tiple MyoD-binding sites. MyoD binds to a con- 
sensus binding sequence of which the apparent 
simplicity belies a rather sophisticated interplay 
among protein, DNA, and subsequent cell type- 
specific transcriptional activation. 
Only 68 amino acid residues of MyoD are re- 
quired for stable muscle cell conversion. This re- 
gion contains the DNA-binding domain, which is 
a putative basic-helix-loop-helix (bHLH) struc- 
ture. There are now well over 30 known bHLH 
proteins, sometimes referred to as the Myc homol- 
ogy family. Extensive mutagenesis has revealed 
that the HLH region is required for dimerization, 
and the adjacent basic region, for DNA binding. 
The Activity of the MyoD Protein 
Is Regulated 
Proliferating myoblasts in tissue culture ex- 
press MyoD RNA and protein, yet do not differen- 
tiate. As assayed by in vivo footprinting, the 
MyoD-binding site in the enhancer of the MCK 
gene is occupied in differentiated myotubes but 
not in myoblasts. Thus there are physiological 
controls that regulate MyoD activity. The deci- 
sion either to proliferate or to differentiate into 
myotubes is governed by the presence or absence 
of serum, but the mechanism of action is un- 
known. One attractive model is that a second type 
of HLH protein is controlling MyoD activity. We 
have been investigating this possibility. 
Information Processing by HLH Proteins 
Several types of biologically important deci- 
sions use information provided by HLH proteins. 
Two of these, MyoD and the achaete-scute prod- 
uct (an HLH protein important for neurogenesis). 
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