Print

Leach, Lynne Heather

Microbiology and Molecular Genetics

2006

PhD

The excretion of the compound pyridine-2,6-dithiocarboxylic acid (PDTC) from three strains of Pseudomonas sp. allows for the fortuitious biodegradation of the environmental contaminant carbon tetrachloride (CT). As the degradation of CT via PDTC does not provide a carbon or energy source for the bacteria, it is important to learn the physiological role of PDTC to better optimize biodegradation of CT in the field. The chemistry of PDTC suggested that its primary physiological role was that of a ferric iron chelator, or siderophore. Receptor-mediated uptake is a hallmark of siderophore internalization. The pdt gene cluster contains genes for both an outer membrane receptor (pdtK) and an inner membrane permease (pdtE). ⁵⁵Fe uptake assays determined that both PdtK and PdtE are essential for the uptake of PDTC-bound iron, conclusively showing PDTC is a siderophore. In addition to facilitating iron assimilation, PDTC is also important in zinc incorporation in P. putida DSM 3601. Zn:PDTC functioned to alleviate a growth inhibition observed in the presence of 1,10-phenathroline, a divalent transition metal chelator. Transcriptional up-regulation of siderophore gene clusters is often triggered by the presence of the cognate siderophore, which induces a tranporter-initiated signaling cascade. As the presence of PDTC is known to up-regulate transcription of the pdt gene cluster, the receptor proteins were tested for a role in signal transduction. Based on a transcriptional reporter assay, neither PdtK nor PdtE were necessary for transcriptional regulation of the pdt gene cluster. The data leave in question the pathway used for the up-regulation of the pdt gene cluster in response to PDTC.