A tripeptide sequence within the nascent DaaP protein is required for mRNA processing of a fimbrial operon in Escherichia coli

The biogenesis of F1845 fimbriae, a member of the Dr family of Escherichia coli adhesins, is regulated by endonucleolytic cleavage of the daaABCDPE primary transcript and differential stability of the resulting cleavage products. Processing of daa mRNA is dependent upon translation of a small open reading frame, designated daaP, which flanks the daa processing site. Here, we demonstrate that daa mRNA processing is directly coupled to daaP translation. Cleavage of the daaA-E mRNA was shown to require the tripeptide Gly-Pro-Pro (GPP), encoded by daaP codons 49-51 downstream of the processing site. Processing also required active translation through RNA located upstream of the processing site; however, processing did not depend on the amino acid sequence encoded by the region of daaP upstream of the processing site. Finally, determination of the processing site was shown to involve its location relative to the codons encoding the GPP tripeptide. These data show that translation of daaP is required in cis to promote RNA processing. These data suggest a model involving interaction of the nascent GPP tripeptide portion of the DaaP polypeptide with the ribosome, triggering cleavage of the associated mRNA at a fixed distance upstream. A model of active involvement of the ribosome in this process is proposed.