Affiliation:
1. Fachbereich Chemie/Biochemie, Philipps-Universität Marburg, Germany.
Abstract
The D-phenylalanine-activating enzyme tyrocidine synthetase I (TycA) from Bacillus brevis ATCC 8185 was overexpressed in Escherichia coli, purified to homogeneity, and assayed for ATP-PPi exchange and covalent binding of phenylalanine by the thiotemplate mechanism. Amino acid exchanges in four different cores of TycA created by site-directed mutagenesis revealed the amino acid residues involved in aminoacyladenylate formation and in covalent thioester formation. Mutations in the putative ATP-binding site SGTTGKPKG caused a decreased phenylalanine-dependent ATP-PPi exchange activity to 10% of the wild-type level for a Lys-186-to-Arg substitution and an almost complete loss of activity (< 1%) for a Lys-186-to-Thr exchange. Alteration of Asp-401 to Asn in the ATPase motif TGDL of TycA decreased the phenylalanine-dependent ATP-PPi exchange activity to 75% of wild type, while an Asp-401-to-Ser mutation decreased the activity to 10% of the wild-type level. Replacement of Ser-562 in the putative thioester-binding motif LGGDSI to Ala or Gly caused a reduction in trichloroacetic acid-precipitable TycA-[14C]phenylalanine complex to one-third of the wild-type level. However, no cleavable [14C]phenylalanine could be detected after treatment with performic acid, indicating that the resulting mutant was unable to form thioester with phenylalanine. In E. coli, TycA was labeled with beta-[3H]alanine, a precursor of 4'-phosphopantetheine, indicating that TycA is modified with a beta-alanine-containing cofactor.
Publisher
American Society for Microbiology
Subject
Molecular Biology,Microbiology
Cited by
114 articles.
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