TRANSCRIPTIONAL ANALYSIS AND REGULATION OF THE SFA DETERMINANT CODINGFOR S-FIMBRIAE OF PATHOGENIC ESCHERICHIA-COLI STRAINS

The sfa determinant codes for S fimbrial adhesins which constitute adh erence factors of pathogenic Escherichia coli strains. We have recentl y shown that the sfa determinant is transcribed from three promoters, pA, pB, and pC. In comparison with the promoters pB and pC, promoter p A, which is located in front of the structural gene sfaA, showed very weak activity. Here we have determined the exact positions of the mRNA start points by primer extension studies. We have also shown that mRN As of 500, 700 and 1400 bases can be detected using oligonucleotide pr obes specific for the genes sfaB, sfaC and sfaA. SfaB and SfaC are pos itive regulators influencing fimbriation and the production of the S-s pecific adhesin which is encoded by the gene sfaS located in the dista l half of the determinant. In addition, it is demonstrated that SfaB a nd SfaC interfere with the regulatory effect of the histone-like prote in H-NS, encoded by a locus termed drdX or osmZ. In a drdX+ strain the regulators are necessary for transcription of the sfa determinant. In contrast, sfa expression is activator-independent in a drdX- strain. In this latter genetic background, a substantial fraction of the sfa t ranscripts is initiated from promoter pA. On the basis of these data w e discuss a model for the regulation of this adhesin-specific determin ant.