Two for One in Iron Homeostasis

Science Signaling  03 Jul 2012:
Vol. 5, Issue 231, pp. ec178
DOI: 10.1126/scisignal.2003356

Chlamydia trachomatis causes a common sexually transmitted infection and blindness. The ytgABCD operon encodes proteins implicated in the regulation of iron homeostasis in these bacteria. In particular, YtgCR is a protein with two apparent functional domains: The N terminus functions as a transmembrane metal permease (called YtgC), and the C terminus is homologous to metal-dependent transcriptional repressors (called YtgR). Thompson et al. showed that this membrane-attached transcriptional repressor was cleaved during infection, exhibited iron-induced repressor activity at the ytg operon, and thus could function in the transcriptional regulation of iron homeostasis. Bioinformatic analysis and molecular modeling indicated that YtgR had conserved iron-coordinating residues and adopted a structure for DNA binding and dimerization similar to other known iron-regulated repressors. In vitro DNA-binding assays with recombinant YtgR produced in Escherichia coli showed that YtgR exhibited iron-stimulated DNA binding. Infection of mammalian cells with C. trachomatis resulted in the appearance of two bands (one full-length and one smaller band) that reacted with YtgCR-specific antibodies, consistent with cleavage occurring during infection. The C-terminal cleavage product was also detected in YtgCR expressed in Escherichia coli. Introduction of YtgCR- or YtgR-encoding plasmids into E. coli, along with a reporter gene controlled by the ytg promoter, confirmed that YtgR produced a functional repressor even when initially expressed as part of the full-length protein. Thus, YtgCR is a dual-function protein that is cleaved to produce a protein that controls iron transport across the membrane and another that controls transcriptional responses necessary to maintain iron homeostasis.

C. C. Thompson, S. S. Nicod, D. S. Malcolm, S. S. Grieshaber, R. A. Carabeo, Cleavage of a putative metal permease in Chlamydia trachomatis yields an iron-dependent transcriptional repressor. Proc. Natl. Acad. Sci. U.S.A. 109, 10546–10551 (2012). [Abstract] [Full Text]