Selection and analysis of non-interactive mutants in the Escherichia coli tryptophan synthase alpha subunit

Linkage map of Escherichia coli K-12, edition 10: the traditional map

This map is an update of the edition 9 map by Berlyn et al. (M. K. B. Berlyn, K. B. Low, and K. E. Rudd, p. 1715-1902, in F. C. Neidhardt et al., ed., Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed., vol. 2, 1996). It uses coordinates established by the completed sequence, expressed as 100 minutes for the entire circular map, and adds new genes discovered and established since 1996 and eliminates those shown to correspond to other known genes. The latter are included as synonyms. An alphabetical list of genes showing map location, synonyms, the protein or RNA product of the gene, phenotypes of mutants, and reference citations is provided. In addition to genes known to correspond to gene sequences, other genes, often older, that are described by phenotype and older mapping techniques and that have not been correlated with sequences are included.

A thermodynamic analysis of conformational change due to the alpha 2 beta 2 complex formation of tryptophan synthase

A characteristic property of the tryptophan synthase alpha 2 beta 2 complex is the mutual activation of the alpha and beta subunit upon complex formation. It has been speculated that this mutual activation results from the conformational change due to the alpha/beta subunit interaction. To elucidate this mechanism, we investigated the thermodynamic parameters of association for the various combinations of the alpha and beta subunits from Escherichia coli and Salmonella typhimurium using isothermal titration calorimetry. The negative association enthalpy of the S. typhimurium alpha subunit with the beta subunit from E. coli (or S. typhimurium) was about 20 kJ mol-1 larger than that of the E. coli alpha subunit at 40 degrees C. However, the favorable enthalpy of the S. typhimurium alpha subunit was perfectly compensated by the unfavorable association entropy, therefore, the Gibbs energy of association was similar to that of the E. coli alpha subunit. Furthermore, the site-directed mutagenesis study revealed that a single mutation (K109N; [Asn109] alpha subunit) of the E coli alpha subunit at the subunit interface from E. coli to the S. typhimurium type could change the characteristics of the thermodynamic parameters of association to the S. typhimurium alpha subunit type. The heat-capacity changes of the association of the alpha subunit with the beta subunit were quite great, 6.37-8.21 kJ mol-1 K-1, compared with that due to a decrease in accessible surface area in the subunit interface. The analysis of the thermodynamic parameters of association suggested that the complex formation couples with the folding (rearrangements) of the alpha subunit monomer or/and beta subunit dimer.

On the role of helix 0 of the tryptophan synthetase alpha chain of Escherichia coli

The role of helix 0 of the alpha chain (TrpA) of the tryptophan synthetase alpha2beta2 multi-functional enzyme complex of Escherichia coli was examined by deleting amino-terminal residues 2-6, 2-11, or 2-19 of TrpA. Selected substitutions were also introduced at TrpA positions 2-6. The altered genes encoding these polypeptides were overexpressed from a foreign promoter on a multicopy plasmid and following insertion at their normal chromosomal location. Each deletion polypeptide was functional in vivo. However all appeared to be somewhat more labile and insoluble and less active enzymatically than wild type TrpA. The deletion polypeptides were overproduced and solubilized from cell debris by denaturation and refolding. Several were partially purified and assayed in various reactions in the presence of tryptophan synthetase beta2 (TrpB). The purified TrpADelta2-6 and TrpADelta2-11 deletion polypeptides had low activity in both the indole + serine --> tryptophan reaction and the indoleglycerol phosphate + serine --> tryptophan reaction. Poor activity in each reaction was partly due to reduced association of TrpA with TrpB. The addition of the TrpA ligands, alpha-glycerophosphate or indoleglycerol phosphate, during catalysis of the indole + serine --> tryptophan reaction increased association and activity. These findings suggest that removal of helix 0 of TrpA decreases TrpA-TrpB association as well as the activity of the TrpA active site. Alignment of the TrpA sequences from different species indicates that several lack part or all of helix 0. In some of these polypeptides, extra residues at the carboxyl end may substitute for helix 0.