STUDIES ON THE FUNCTION OF CONSERVED SEQUENCE MOTIFS IN THE T4 DAM-[N-6-ADENINE] AND ECORII [C5-CYTOSINE] DNA METHYLTRANSFERASES

We used site-directed oligodeoxyribonucleotide-mediated mutagenesis an d kinetic studies with purified wild-type (wt) and mutant proteins to evaluate the role of the conserved sequence motifs in two prokaryotic DNA MTases. We suggest that: (i) the main role of Pro in the M . EcoRI I PC-motif is to restrict the conformational freedom of Cys and orient it in a manner essential for catalysis; (ii) in both M EcoRII and T4 Dam the FXGXG-motif positions AdoMet with respect to the catalytic sit e; (iii) the DPPY-motif in T4 Dam (region IV) is important for AdoMet- binding and may be part of the binding site; and (ia) the RXNXKXXFXXPF K-motif in T4 Dam (region III) is part of the DNA binding/recognition domain.