Positive selection for loss-of-function tat mutations identifies critical residues required for TatA activity.

The Tat system, found in the cytoplasmic membrane of many bacteria, is a general export pathway for folded proteins. Here we describe the development of a method, based on the transport of chloramphenicol acetyltransferase, that allows positive selection of mutants defective in Tat function. We have demonstrated the utility of this method by selecting novel loss-of-function alleles of tatA from a pool of random tatA mutations. Most of the mutations that were isolated fall in the amphipathic region of TatA, emphasizing the pivotal role that this part of the protein plays in TatA function.