PdhR, the pyruvate dehydrogenase repressor, does not regulate lipoic acid synthesis

Area of Science:

• Molecular Biology
• Biochemistry
• Microbiology

Background:

• Lipoic acid is a vital enzyme cofactor for central metabolism across all life.
• The lipoic acid synthesis pathway in Escherichia coli has been extensively studied.
• Previous research suggested the pyruvate dehydrogenase repressor (PdhR) regulates lipA transcription.

Purpose of the Study:

• To investigate the regulatory role of PdhR on lipA gene expression in Escherichia coli.
• To validate or refute the previously identified PdhR binding site (lipA site 1) upstream of the lipA gene.
• To determine the physiological relevance of PdhR in lipoic acid biosynthesis regulation.

Main Methods:

• Purification of E. coli PdhR protein.
• Electrophoretic mobility shift assays (EMSA) to assess PdhR binding to DNA sequences.
• Construction and testing of modified PdhR binding sites (lipA site 3 palindrome).
• Transcriptional fusion of lipA promoter variants to a LacZ reporter gene.
• Assay of beta-galactosidase activity in pdhR deletion mutants.

Main Results:

• The previously proposed lipA site 1 did not bind purified PdhR in vitro.
• PdhR binding was observed at a distant site, lipA site 3, which is a weak half-palindrome.
• Engineering lipA site 3 into a palindrome significantly increased PdhR binding affinity.
• Deletion of the pdhR gene did not alter lipA expression levels, regardless of the promoter construct.

Conclusions:

• PdhR does not physiologically regulate the transcription of the lipA gene in Escherichia coli.
• The putative regulatory site identified in prior work is nonfunctional and irrelevant.
• Lipoic acid gene expression is not controlled by PdhR, contrary to previous hypotheses.