The binding of MC1 protein, the major chromosomal protein of the archaebacterium Methanosarcina sp. CHTI 55, to the region proceeding the strongly expressed genes encoding methyl co-enzyme reductase in a closely related micro-organism has been investigated. By gel retardation and DNAase I footprinting assays, we identified a preferential binding sequence in a open reading frame of unknown function. The large area of DNA protected against DNAase I is interrupted by a strong cleavage enhancement site on each strand. By circular permutation assays, we showed that the DNA bends upon MC1 binding. Furthermore we observed that the presence of a sequence outside the binding site can induce an unusual electrophoretic behaviour in some complexes.