Figure 1.

Pathway of Moco Biosynthesis in Plants.

The proposed starting compound GTP, the intermediates precursor Z, and the metal binding MPT as well as the end product Moco are shown. Cnx2 and Cnx3 (Hoff et al., 1995) are involved in opening the pyrazine ring of guanine in a cyclohydrolase-like reaction (Wuebbens and Rajagopalan, 1995), forming the sulfur-free precursor Z (Wuebbens and Rajagopalan, 1993). In the second step, sulfur has to be introduced to form the dithiolene group. This step is catalyzed by MPT synthase, a complex of Cnx6 and Cnx7 (Mendel and Schwarz, 1999; R.R. Mendel, unpublished result) that is resulfurated by the sulfurase Cnx5 (Nieder et al., 1997). Finally, Mo is activated and incorporated into MPT by the activity of Cnx1 (Stallmeyer et al., 1995).