Structural Studies on DNA-Replication Enzymes, src- related Oncogene 
Products, and Oxidoreductases 
of a complex of SH2 with a phosphotyrosyl pep- 
tide has been completed at 1.5 -A resolution. 
Redox Proteins and the Transcriptional 
Response to Oxidative Stress* 
We have recently solved and refined the three- 
dimensional structures of two related redox en- 
zymes, thioredoxin reductase and trypanothione 
-reductase. Crystallographic investigations of 
enzyme-substrate complexes are now in progress 
(G. Waksman). Work has also begun on a new 
member of the disulfide reductase family. Pro- 
teins related to thioredoxin have been implicated 
in the process by which disulfide-containing pro- 
teins fold up rapidly without scrambling their 
disulfide pairings. 
James Bardwell, Karen McGovern, and Jon 
Beckwith (Harvard Medical School) have re- 
cently identified an E. colt protein that is re- 
quired for the correct folding of disulfide- 
containing proteins in vivo. This 21-kDa protein, 
the product of the dsbA gene, has no detectable 
sequence similarity to any known protein except 
for a short stretch of amino acids with similarity 
to the active site of thioredoxin (including the 
redox-active disulfide bond). The dsbA protein 
has been purified in our laboratory, from cells 
provided by Bardwell and Beckwith, and single 
crystals that diffract to 2-A resolution have 
been obtained. The determination of the three- 
dimensional structure is under way, and we are 
also pursuing the construction of site-specific 
mutants at the active site (Jennifer Martin) . 
We are interested in determining the mecha- 
nism of the bacterial oxidative stress sensor 
OxyR. The OxyR protein responds to oxidizing 
agents by elevating the expression of such redox 
enzymes as catalase, superoxide dismutase, and 
glutathione reductase. The intact OxyR molecule 
proved to be unstable in the absence of DNA. In 
collaboration with Gisela Storz (National Insti- 
tutes of Health), Scott Robertson in our labora- 
tory has cloned, overexpressed, and purified the 
regulatory domain alone, and its crystallization is 
being pursued. 
' This work is being supported by a grant from the National 
Institutes of Health. 
Representations of the structure of 
fi-subunit — the sliding clamp — of 
DNA polymerase III of Escherichia 
coli, determined by x-ray crystallog- 
raphy. The atomic structure is 
shown in yellow and its electrostatic 
potentials in red ( negative ) and 
blue (positive). The top two images 
are different views of the dimer, and 
the bottom is a single monomer. The 
electrostatic potential generated by 
the protein helps stabilize the dimer 
and its interactions with the DNA 
molecule ( not shown ), which 
threads through the clamp formed 
by the protein. 
From Kong, X.-P., Onrust, R., 
O'Donnell, M., and Kuriyan, f. 1992. 
Cell 69:425-437. Copyright© 1992 
by Cell Press. 
236 
