Signal peptide etiquette during assembly of a complex respiratory enzyme

Salmonella enterica serovar Typhimurium is a Gram-negative pathogen capable of respiration with a number of terminal electron acceptors. Tetrathionate reductase is important for the infection process and is encoded by the ttrBCA operon where TtrA and TtrB are metallocofactor-containing proteins targeted to the periplasmic side of the membrane by two different Tat targeting peptides. In this work, the inter-relationship between these two signal peptides has been explored. Molecular genetics and biochemical approaches reveal that the processing of the TtrB Tat signal peptide is dependent on the successful assembly of its partner protein, TtrA. Inactivation of either the TtrA or the TtrB Tat targeting peptides individually was observed to have limited overall effects on assembly of the enzyme or on cellular tetrathionate reductase activity. However, inactivation of both signal peptides simultaneously was found to completely abolish physiological tetrathionate reductase activity. These data suggest both signals are normally active during assembly of the enzyme, and imply a code of conduct exists between the signal peptides where one can compensate for inactivity in the other. Since it appears likely that tetrathionate reductase presents itself for export as a multi-signal complex, these observations also have implications for the mechanism of the bacterial Tat translocase.