Structure and Function of RNA Polymerase II 
phorylation is known. We have used synthetic 
CTD peptide repeats as substrates to identify en- 
zymes (CTD kinases) that carry out this phos- 
phorylation. Two CTD kinases have been iso- 
lated, each containing protein kinase catalytic 
subunits previously identified in yeast as cell- 
division control proteins. The identification of a 
cell cycle control protein as a component of an 
enzyme that modifies RNA polymerase suggests 
that cell growth may be controlled, at least in 
part, through phosphorylation of the CTD. 
Using the purified CTD kinases, we have 
mapped the sites of phosphorylation on the CTD. 
These sites, serines that precede proline residues, 
correspond to sites in other proteins that are mod- 
ified in a cell cycle-dependent fashion. Current 
studies are directed toward examining the level 
of CTD phosphorylation at different times in the 
cell cycle. One consequence of CTD phosphory- 
lation is that the CTD becomes greatly extended. 
The basis of this conformational change is also an 
area of investigation in our laboratory. 
To define more precisely the amino acid se- 
quences required for CTD function, we have de- 
vised a strategy to clone synthetic CTD repeats. 
This work is being done in the yeast Saccharo- 
myces cerevesiae, where as few as 1 0 of the re- 
peats are sufficient for viability. We have con- 
structed mutant CTDs in which the residues 
identified as phosphorylation sites have been 
changed. In most cases these mutations are lethal, 
indicating that phosphorylation is essential for 
RNA polymerase II function. We are currently us- 
ing yeast genetics to isolate suppressors of these 
mutations, in an effort to identify proteins that 
interact with the CTD. Through characterization 
of such proteins we hope to determine the func- 
tion of the CTD in RNA polymerase II function. 
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