Transcription Factor Interactions 
speculation provided a satisfying fit between the 
dimeric proteins and their DNA-binding sites. 
A common property shared by the binding sites 
for leucine zipper proteins is dyad symmetry. 
Two short sequences, related to each other as are 
the symmetric halves of a restriction endonucle- 
ase recognition site, are directly abutted. The ba- 
sic region of one polypeptide chain would fit one 
half of the dyad symmetric binding site, while 
that of the other chain would fit the other half. 
Evidence has recently been obtained by a number 
of different laboratories in support of this idea. 
The acid test, however, will come upon resolu- 
tion of the actual molecular structures of leucine 
zipper proteins. Paul Sigler (HHMI, Yale Univer- 
sity) and Stephen Harrison (HHMI, Harvard Uni- 
versity) are well on the way to crystallizing two 
different leucine zipper proteins. 
308 
