Recombinant Chlamydophila caviae Tryptophan synthase alpha chain (trpA)

Genomic Context and Operon Organization

Chlamydophila caviae (formerly Chlamydia psittaci) harbors a near-complete tryptophan biosynthesis pathway. Its trp operon includes:

• TrpA: Alpha chain of tryptophan synthase (catalyzes the α-reaction)

• TrpB: Beta chain (catalyzes the β-reaction)

• TrpD/F/C: Enzymes for chorismate-to-tryptophan conversion

• TrpR: Tryptophan-dependent transcriptional regulator

This organization contrasts with C. trachomatis, which retains only trpR and trpBA (with non-functional TrpA) .

Table 1: Comparison of Trp Operon Genes in Chlamydophila caviae vs. Chlamydia trachomatis

Catalytic Activity

In C. caviae, TrpA retains full enzymatic activity for the α-reaction, enabling conversion of indole-3-glycerol phosphate to indole and glyceraldehyde-3-phosphate. This activity is absent in C. trachomatis TrpA due to mutations in the active site .

Subunit Interactions

Recombinant TrpA forms a tetrameric complex (α₂β₂) with TrpB. Structural studies suggest that TrpA’s β-loop L6 (residues 176–196) and C-terminal regions (e.g., Phe22, Glu49, Asp60) are critical for substrate binding and allosteric regulation .

Table 2: Key Residues in C. caviae TrpA

Transcriptional Control

The trp operon in C. caviae is regulated by TrpR, a tryptophan-dependent aporepressor. TrpR binds to operator sequences upstream of trpRBA in the presence of tryptophan, repressing transcription . This system allows C. caviae to adapt to changing tryptophan availability.

Recombinant Production and Applications

Recombinant TrpA from C. caviae has been expressed in heterologous systems (e.g., E. coli) to study its catalytic and structural properties. Key findings include:

• Enzymatic Activity: TrpA converts indole-3-glycerol phosphate to indole with kinetic parameters comparable to E. coli TrpA .

• Allosteric Regulation: TrpA modulates TrpB activity, even in C. trachomatis, suggesting conserved regulatory roles .

Table 3: Kinetic Parameters of Recombinant TrpA

Genetic Recombination

In C. trachomatis, lateral gene transfer (LGT) has generated recombinants with altered TrpA, impacting tryptophan synthesis and host adaptation . While C. caviae retains a functional TrpA, LGT events may influence its evolution, particularly in regions like the β-loop L6 .

Host Adaptation

The presence of kynU and prsA in C. caviae enables metabolic flexibility, allowing tryptophan synthesis despite host immune responses (e.g., IFN-γ-mediated tryptophan depletion) . This contrasts with C. trachomatis, which relies on host indole supplementation .

Research Gaps and Future Directions

1. Recombinant TrpA Studies: Limited data exist on C. caviae TrpA’s crystal structure or interaction with TrpB.

2. Pathogenic Role: The contribution of TrpA to C. caviae virulence remains unexplored.

3. Therapeutic Targets: TrpA’s catalytic activity in C. caviae may offer novel targets for antimicrobial development.