Nucleotide sequence of the Neurospora crassa trp-3 gene encoding tryptophan synthetase and comparison of the trp-3 polypeptide with its homologs in Saccharomyces cerevisiae and Escherichia coli

The complete nucleotide sequence of the Neurospora crassa trp-3 gene-encoding tryptophan synthetase has been determined; we present an analysis of its structure. A comparison of the deduced amino acid sequence of the trp-3 polypeptide with its homologs in Saccharomyces cerevisiae (encoded by the TRP5 gene) and Escherichia coli (encoded by the trpA and trpB genes) shows that the A and B domains (amino acid segments homologous to the trpA and trpB polypeptides, respectively) of the N. crassa and yeast polypeptides are in the same order (NH2-A-B-COOH). This arrangement is the reverse of the gene order characteristic of all prokaryotes that have been examined. N. crassa tryptophan synthetase has strong homology to the yeast TRP5 polypeptide (A domains have 54% identity; B domains have 75% identity), and somewhat weaker homology to the E. coli trpA and trpB polypeptides (A domains have 31% identity; B domains have 50% identity). The two domains of the N. crassa polypeptide are linked by a connector of 54-amino acid residues that has less than 25% identity to the 45-residue connector of the yeast polypeptide, although secondary structure analysis predicts both connectors would be alpha-helical. In contrast to the yeast TRP5 gene, which has no introns, the trp-3 coding region is interrupted by two introns 77 and 71 nucleotides in length. Both introns are located near the 5'-end of the gene and therefore not near the segment encoding the connector.