TatD is a cytoplasmic protein with DNase activity - No requirement for TatD family proteins in Sec-independent protein export

The Escherichia coli Tat system mediates Sec-independent export of protein precursors bearing twin arginine signal peptides. Genes known to be involve d in this process include tafA, tatB, and tatC that form an operon with a f ourth gene, tatD. The tatD gene product has two homologues in E. coli coded by the unlinked ycfH and yjjV genes. An E. coli strain with in-frame chrom osomal deletions in all three of tatD, ycfH, and yjjV exhibits no significa nt defect in the cellular location of five cofactor-containing enzymes that are synthesized with twin arginine signal peptides. Neither these mutation s nor overproduction of the TatD protein cause any discernible effect on th e export kinetics of an additional E. coli Tat pathway substrate. It is con cluded that proteins of the TatD family have no obligate involvement in pro tein export by the Tat system. TatD is shown to be a cytoplasmic protein. T atD binds to immobilized Ni2+ or Zn2+ affinity columns and exhibits magnesi um dependent DNase activity. Features of the tafA operon that may control T atD expression are discussed.