The Tat system catalyzes the transport of folded globular proteins across the bacterial plasma membrane and the chloroplast thylakoid. It recognizes cleavable signal peptides containingacritical twin-argininemotif but little isknownof the overall structure of these peptides. In this report, we have analyzed the secondary structure of the SufI signal peptide, together with those of two nonfunctional variants in which the region around the twin-arginine, RRQFI, is replaced by KKQFI or RRQAA. Circular dichroism studies show that the SufI peptide exists as anunstructuredpeptide in aqueous solvent with essentially no stable secondary structure
