Preliminary crystallographic analysis of a novel type-II thiol transferase from Methanocaldococcus jannaschii. Methanocaldococcus jannaschii produces a novel thiol transferase, MjHT, that carries out an unusual, type II catalytic mechanism by requiring a covalently linked cofactor that was identified to be pyridoxal 5'-phosphate (PLP). Crystals of MjHT have been prepared and diffraction data collected to 1.1 A resolution using synchrotron radiation. A crystal form suitable for X-ray analysis was grown in the presence of the substrate analog 2-mercaptoethanol as well as the reaction product alanine, and an additional diffraction data set was collected at 2.6 A resolution to investigate the electron density in this region. This study, therefore, provides the first complete electron density map of any member of this class of enzymes and demonstrates a novel catalytic mechanism that uses an amino-acid side chain as a nucleophile. MjHT is the first thiol transferase for which a complete structure is now available for this group of enzymes. We present here a detailed picture of MjHT structure that may also provide some clues for the understanding of this entire group of enzymes.