Reduced Environmental Redox Potential Affects Both Transcription and Expression of the Pap Pili Gene

Abstract

Pyelonephritis-associated pili (pap) gene expression is subject to a phase variation control mechanism by which cells alternate between two pili-expression states, viz. a 'phase-off (pili - ) and a 'phase-on' (pili + ) state. During interaction with a host, Escherichia coli encounter various environmental redox conditions. We have addressed the question of whether bacteria are able to respond to this environmental signal by regulating pap pili biogenesis, a crucial colonisation factor in pyelonephritis. Transcription from the PapB promoter (papBAp) was studied in the Salmonella typhimurium papBAp lac fusion lysogen strain under aerobic, microaerobic and anaerobic conditions. In this strain, the β-galactosidase gene is under the control of the papB promoter that initiates transcription of both the papB gene encoding the regulatory papB protein and the papA gene encoding the structural pilin protein. The frequency of switching rrom the Lac + (papBAp 'on') to the Lac - . (papBAp 'off) state was about 1-3-fold higher when the environmental redox potential was reduced by changing from aerobic to microaerobic and anaerobic growth milieus. The β-galactosidase activity representing the rate of transcriptional initiation from the papB promoter was, as calculated pcr 10 8 Lac + bacteria, more than 12-fold higher in acrobically cultivated bacteria than in bacteria cultured undcr microaerobic or anaerobic conditions. Pap pili adhesin expression was measured under the same redox conditions, using E. coli K12 HB101 pPap 5 containing a plasmid coding for whole pap pili operon. The strongest pap pili expression, measured as agglutination of latex gal-gal beads, was observed under microaerobic conditions similar to those found in the urinary tract. Under anaerobic conditions like those prevalent in the intestine, pap pili expression was negligible. This is not surprising, since such expression would not represent an ecological advantage for E. coli. In fact, repression of these types of fimbriae under anaerobic conditions may be a way in which the bacteria can save energy which can then be used to promote growth. Although the two genetic models used for transcription and expression studies are distinct, a high rate of transcription did not seem to correlate with optimal pili expression. This may indicate the importance of the post-transcriptional processing in pap pili expression. Keywords: Pilus gene regulation; Growth conditions; Aerobic; Anaerobic; Microaerobic; Adhesion.