MUTATIONS AFFECTING MESSENGER-RNA PROCESSING AND FIMBRIAL BIOGENESIS IN THE ESCHERICHIA-COLI PAP OPERON

The Escherichia coli pnp genetic determinant includes 11 genes and enc odes expression of Pap pill on the bacterial surface. An RNase E-depen dent mRNA-processing event in the intercistronic papB-papA region resu lts in the accumulation of a papA-gene-specific mRNA in considerable e xcess of the primary papB-papA mRNA transcription product. We have int roduced mutations in the intercistronic region and studied the effect in vivo of these mutations on the processing event, PapA protein expre ssion, and the biogenesis of fimbriae on the bacterial surface. Our st udies establish that mRNA processing is an important event in the mech anism resulting in differential gene expression of the major pap opero n. The deletion of sequences corresponding to the major cleavage site abolished processing, reduced expression of PapA protein, and resulted in ''crew-cut'' bacteria with short fimbrial structures on the bacter ial surface. Only a limited part of the intercistronic region appeared to be required as the recognized target for the processing to occur. Upstream sequences to a position within 10 nucleotides of the major RN ase E-dependent cleavage site could be deleted without any detectable effect on papB-papA mRNA processing, PapA protein expression, or fimbr ia formation. Substitution mutations of specific bases at the cleavage site by site-directed mutagenesis showed that there were alternative positions at which cleavage could be enhanced, and tests with an in vi tro processing assay showed that such cleavages were also RNase E depe ndent. Our findings are discussed in relation to other fimbrial operon s and other known targets of the RNase E endoribonuclease.