Molybdopterin Dinucleotide Biosynthesis in Escherichia coli IDENTIFICATIONOFAMINOACIDRESIDUESOFMOLYBDOPTERINDINUCLEOTIDE TRANSFERASESTHATDETERMINESPECIFICITYFORBINDINGOFGUANINEORCYTOSINE NUCLEOTIDES

The molybdenum cofactor is modified by the addition ofGMP or CMP to the C4 (i) formation of precursor Z (3, 4); (ii) formation of MPT fromphosphate of molybdopterin formingthe molybdopterin guanine dinucleotide or molybdopterincytosine dinucleotide cofactor, respectively. The two reactionsare catalyzed by specific enzymes as follows: the GTP:molyb-dopterin guanylyltransferase MobA and the CTP:molybdopt-erin cytidylyltransferase MocA. Both enzymes show 22%amino acid sequence identity and are specific for their respec-tive nucleotides. Crystal structure analysis of MobA revealedtwo conserved motifs in the N-terminal domain of the proteininvolved in binding of the guanine base. Based on these motifs,we performed site-directed mutagenesis studies to exchangethe amino acids to the sequence found in the paralogue MocA.Using a fully defined