SALMONELLA-ENTERITIDIS AGFBAC OPERON ENCODING THIN, AGGREGATIVE FIMBRIAE

Salmonella enteritidis produces thin, aggregative fimbriae, named SEF1 7, which are composed of polymerized AgfA fimbrin proteins. DNA sequen ce analysis of a 2-kb region of S. enteritidis DNA revealed three cont iguous genes, agfBAC. The 453-bp agfA gene encodes the AgfA fimbrin, w hich was predicted to be 74% identical and 86% similar in primary sequ ence to the Escherichia coli curli structural protein, CsgA. pH;IG, a pUC18 derivative containing a 3.0-kb HindIII fragment encoding agfBAC, directed the in vitro expression of the major AgfA fimbrin, with an M (r) of 17,000, and a minor AgfB protein, with an M(r) of 16,000, encod ed by the 453-bp agfB gene. AgfA was not expressed from pDAG, a pUC18 derivative containing a 3.1-kb DraI DNA fragment encoding agfA but not agfB. Primer extension analysis identified two adjacent transcription start sites located immediately upstream of agfB in positions analogo us to those of the E. coli curlin csgBA operon. No transcription start sites were located immediately upstream of agfA or agfC. Northern (RN A) blot analysis confirmed that transcription of agfA was initiated fr om the agfB promoter region. Secondary-structure analysis of the putat ive mRNA transcript for agfBAC predicted the formation of a stem-loop structure (Delta G(o), -22 kcal/mol [-91 kJ/mol]) in the intercistroni c region between agfA and agfC, which may be involved in stabilization of the agfBA portion of the agfBAC transcript. agfBAC and flanking re gions had a high degree of sequence similarity with those counterparts of the E. coli curlin csgBA region for which sequence data are availa ble. These data are demonstrative of the high degree of similarity bet ween S. enteritidis SEF17 fimbriae and E. coli curli with respect to f imbrin amino acid sequence and genetic organization and, therefore, ar e indicative of a common and relatively recent ancestry.