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9.A.56 The Proposed Outer Membrane Anion Porin, TsaT (TsaT) Family

Inducible mineralization of 4-toluenesulphonate (TSA) by Comamonas testosteroni T-2 is initiated by a secondary transport system, followed by oxygenation and oxidation by TsaMBCD to 4-sulphobenzoate under the regulation of TsaR and TsaQ. Mampel et al. (2004) presented evidence for a novel, two-component transport system (TsaST). TsaT was reported to be an outer-membrane porin forming an anion-selective channel that works in co-operation with the secondary transporter, TsaS, (9.A.29.1.1) located in the inner membrane. tsaT was identified as a 1017-bp ORF (open reading frame) on plasmid pTSA upstream of the TSA-catabolic genes in the tsa operon. Expression of tsaT was regulated by TsaR, the transcriptional activator of the tsa regulon. The presence of tsaT was concomitant with the presence of the tsa operon in different TSA-degrading isolates. tsaT was expressed in Escherichia coli and was detected in the outer membrane. A 22-amino-acid leader peptide was identified. The purified protein was reconstituted in lipid bilayer membrane and was reported to form anion-selective channels with a single-channel conductance of 3.5 nS in 1 M KCl. Downstream of tsaT, a constitutively expressed 720-bp ORF (tsaS) was identified. tsaS codes for a hydrophobic protein which is most likely localized in the cytoplasmic membrane. tsaS is adjacent to tsaT, but showed a different transcriptional profile (Mampel et al., 2004).

TsaT is homologous to (32% identity) to solute binding proteins of the DctP family (Trap-T systems) (P37735; TC# 2.A.56.1.1). Therefore, its assignment as an outer membrane porin is in doubt. It is in the DctP or SBP_bac_7 family. (M. Saier, unpublished observation)

The proposed generalized transport reaction provided by TsaT is:

aromatic acid (out) → aromatic acid (periplasm)