DNA-BINDING INDUCES A MAJOR STRUCTURAL TRANSITION IN A TYPE-I METHYLTRANSFERASE

The type IC DNA methyltransferase M.EcoR124I is a complex multisubunit enzyme that recognizes the nonpalindromic DNA sequence GAAN(6)RTCG. S mall angle X-ray scattering has been used to investigate the solution structure of the methyltransferase and of complexes of the enzyme with unmethylated and hemimethylated 30 bp DNA duplexes containing the spe cific recognition sequence, A major change in the quaternary structure of the enzyme is observed following DNA binding, based on a decrease in the radius of gyration from 56 to 40 Angstrom and a reduction in th e maximum dimension of the enzyme from 180 to 112 Angstrom. The struct ural transition observed is independent of the methylation state of th e DNA, CD shows that there is no change in the secondary structure of the protein subunits when DNA is bound, In contrast, there is a large increase in the CD signal arising from the DNA, suggesting considerabl e structural distortion which may allow access to the bases targeted f or methylation, We propose that DNA binding induces a large rotation o f the two HsdM subunits towards the DNA, mediated by hinge bending dom ains in the specificity subunit HsdS.