Structural studies of the BstVI restriction-modification proteins by fluorescence spectroscopy

Structural studies of the proteins of the BstVI restriction-modification system of Bacillus stearothermophilus V were carried out using intrinsic fluorescence techniques. The exposure and environments of their tryptophanyl residues were determined using collisional quenchers. Quenching of BstVI endonuclease by iodide suggested a heterogeneous class of tryptophan residues, while the results obtained with M.BstVI methylase were consistent with a rather exposed tryptophan population. A comparison of the quenching efficiencies at 20 degrees C and 55 or 60 degrees C showed that their structures are more flexible and open at the temperature at which they exhibit maximal activity. The endonuclease reached its active conformation only after 1 h of incubation at 60 degrees C. Fluorescence changes were observed upon Mn2+ and Mg2+ binding, with Kd values in the range 3-5 microM. The binding of S-adenosyl-L-methionine to the methylase produced conformational changes, which were consistent with binding to a single site of Kd 550 and 680 microM at 20 degrees C and 55 degrees C, respectively. Quenching experiments with iodide showed that the presence of S-adenosyl-L-methionine leads to different conformational states at 20 degrees C and 55 degrees C. These results were interpreted in terms of differences in the structural characteristics of these restriction-modification proteins as well as in terms of differences in the conformational states that these enzymes exhibit at 20 degrees C and at the temperature at which they are most active.