Oxidation-sensitive residues mediate the DNA bending abilities of the architectural MC1 protein.

The Methanosarcina thermophila MC1 protein is a small basic protein that is able to bend DNA sharply. When this protein is submitted to oxidative stress through gamma irradiation, it loses its original DNA interaction properties. The protein can still bind DNA but its ability to bend DNA is decreased dramatically. Here, we used different approaches to determine the oxidations that are responsible for this inactivation. Through a combination of proteolysis and mass spectrometry we have identified the three residues that are oxidized preferentially. We show by site directed mutagenesis that two of these residues, Trp74 and Met75, are involved in the DNA binding. Their substitution by alanine leads to a strong reduction in the protein capacity to bend DNA, and a total loss of its ability to recognize bent DNA. Taken together, these results show that oxidation of both these residues is responsible for the protein inactivation. Furthermore, the results confirm the strong relationship between DNA bending and recognition of DNA sequences by the MC1 protein.