Methanococcus jannaschii has an mtr gene cluster expressing N(5)-methyltetrahydromethanopterin:coenzyme M methyltransferase, which generates methane by reducing CO(2) with H(2) with concomitant energy production under strictly anaerobic conditions. Some methanogenic archaea also have an mtr gene-cluster homologue, the mtxXAH gene cluster. M. jannaschii has both an entire mtr gene cluster and a single mtxX gene instead of the whole mtxXAH gene cluster. A PSI-BLAST search, secondary-structure prediction and the absence of phosphotransacetylase activity in M. jannaschii strongly support the possibility that the MtxX protein constitutes a unique methyltransferase family. In this study, the MtxX protein from M. jannaschii has been cloned, expressed, purified and crystallized. Synchrotron data were collected to 2.9 A from a crystal of selenomethionine-substituted MtxX protein. The crystal belonged to the primitive hexagonal space group P6(1)22, with unit-cell parameters a = 54.9, b = 54.9, c = 341.1 A, beta = 120.0 degrees . A full structure determination is under way in order to provide insight into the structure-function relationship of this protein.