Yersinia yopQ mRNA encodes a bipartite type III secretion signal in the first 15 codons

The type III machinery of Yersinia transports Yop proteins across the bacterial envelope. The minimal secretion signal of yopQ is located in codons 1-10 that, when fused in frame to the neomycin phosphotransferase gene, is sufficient to promote type III secretion of YopQ(1-10)-Npt. Frame-shift mutations, generated by nucleotide insertions or deletions following the AUG start and suppressed at the fusion site with npt, abrogate signalling of yopQ(1-10) but not of yopQ(1-15). By generating transversions of every single nucleotide in yopQ(1-10), we identified 10 nucleotide positions in codons 2, 3, 5, 7, 9 and 10 that were each required for substrate recognition. One transversion that abolishes secretion, uridyl 9 to adenyl (U9A), is a synonymous codon 3 mutation that retains the original amino acid as confirmed by Edman degradation analysis, suggesting that the mRNA but not the amino acid sequence of yopQ(1-10) is involved in secretion signalling. Although transversion of U8A abrogates signalling of yopQ(1-10), fusion of yopQ codons 11-15 restores secretion. The nucleotides that are required for this suppression by yopQ(11-15) were identified and revealed both synonymous and non-synonymous mutations. Frame-shift mutations introduced into just this suppressor region (codons 11-15) did not abrogate its ability to suppress mutations in the minimal secretion signal (codons 1-10). Thus, elements downstream of the minimal secretion signal of YopQ increase the efficiency of YopQ secretion and suppress mutations elsewhere in the secretion signal.