SYRD IS REQUIRED FOR SYRINGOMYCIN PRODUCTION BY PSEUDOMONAS-SYRINGAE PATHOVAR SYRINGAE AND IS RELATED TO A FAMILY OF ATP-BINDING SECRETION PROTEINS

The syrD gene of Pseudomonas syringae pathovar syringae strain B301D-R was characterized and sequenced. The syrD open reading frame is 1695 bp long and encodes a predicted protein, SyrD, of almost-equal-to 63 k Da. Database searches revealed that SyrD shares a high degree of simil arity with the ATP-binding cassette (ABC) superfamily of transporter p roteins which are responsible for specific nutrient uptake and for sec retion of certain cellular products in prokaryotes, and for multiple d rug resistance in mammals. The amino acid sequence homology between Sy rD and the ABC proteins was greatest at the conserved residues which c onstitute the ATP-binding cassette of these proteins; these residues l ie in the hydrophilic C-terminal half of SyrD. The N-terminus of SyrD is predicted to be hydrophobic and to contain six membrane-spanning al pha-helices. syrD mutants of strain B301D-R were significantly less vi rulent than other syr mutants, were deficient in four large-polypeptid es thought to be components of a syringomycin synthetase complex, and showed reduced expression of a syrB-lacZ reporter gene fusion in trans . It is proposed that SyrD is a cytoplasmic membrane protein that func tions as an ATP-driven efflux pump for the secretion of syringomycin.