Ribonuclease H and Genetic Replication 
the nucleic acid, assuming that divalent cations 
are present at the carboxyl triad. 
Based on these observations, we have built a 
model of the complex of RNase H with a hybrid 
duplex in the A conformation. We are attempting 
to cocrystallize the enzyme with oligonucleotide 
hybrids. We have also grown crystals of a com- 
plex with Mg^^ and are in the process of solving 
that structure. It appears that the catalytic mecha- 
nism is most likely to be similar to that of ribo- 
zymes and of the 3'^ 5' exonuclease of DNA poly- 
merase I, which Thomas Steitz (HHMI, Yale 
University) and his co-workers have studied in 
detail. 
Relationships to Retroviral Reverse 
Transcriptase 
The sequences of E. coli and yeast RNases H are 
clearly related to the RNase H portions of retro- 
viral reverse transcriptases. Seven of the nine resi- 
dues that are absolutely conserved among 1 3 re- 
troviral RNases H occur in common with the 
microbial enzymes. Moreover, in a topologically 
restricted alignment, E. co/?' RNase H has 28 per- 
cent overall amino acid sequence identity with 
MMLV RNase H and 24 percent identity with HIV 
RNase H. 
Our sequence alignments also suggest impor- 
tant structural differences among the various 
RNase H proteins. This has led us to an exercise in 
homology modeling in collaboration with Ken- 
neth Smith and Barry Honig at Columbia. It ap- 
pears that HIV RNase H has a 1 5-residue deletion 
relative to E. colt RNase H and that this can 
readily be accommodated. Our comparisons have 
also suggested mutations, which are now being 
studied in Steve Goff's laboratory at Columbia 
with the MMLV reverse transcriptase. 
Finally we have returned to our structural stud- 
ies on the MMLV reverse transcriptase. Millie 
Georgiadis is trying to produce usable crystals 
with new constructs prepared by Alice Teles- 
nitsky in the Goff laboratory. We hope that what 
we have learned from E. co/? RNase H, including 
MAD phasing based on selenomethionine, will 
help in our analysis of the retroviral enzyme. 
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