Constitutive Expression of Escherichia coli tat Genes Indicates an Important Role for the Twin-Arginine Translocase during Aerobic and Anaerobic Growth

The transcription start sites for the tatABCD and tatE loci, encoding components of the Tat (twin-arginine translocase) protein export pathway, have been identified. Expression studies indicate that the tatABCD and tatE transcription units are expressed constitutively. Translational fusion experiments suggest that TatA is synthesized at a much higher level than the other Tat proteins. The Tat (twin-arginine translocation) export pathway is a recently discovered protein transport system found in the cy-toplasmic membranes of most bacteria and in the energy-trans-ducing membranes of plant organelles (2, 17). Proteins are targeted to the Tat apparatus by N-terminal signal sequences that harbor the (S/T)RRxFLK twin-arginine motif (1). Analy-sis of the Escherichia coli genome sequence has identified in excess of 20 gene products that are likely to be exported by the Tat system (2). Unlike the well-characterized Sec export path-way, which translocates substrates in an extended conforma-tion, the Tat pathway appears to be capable of exporting pre