Influence of l -Leucine and l -Alanine on Lrp Regulation of foo , Coding for F165 ₁ , a Pap Homologue

The foo operon encodes F165 1 fimbriae that belong to the P-regulatory family and are synthesized by septicemic Escherichia coli . Using an Lrp-deficient host and the lrp gene cloned under the arabinose pBAD promoter, we demonstrated that foo was transcribed proportionally to the amount of Lrp synthesized. l -Leucine and l -alanine decreased drastically the steady-state transcription of foo and modified phase variation, independently of the presence of FooI. Specific mutations in the C-terminal region of Lrp reduced or abolished the repressive effect of these amino acids, indicating that they modulate F165 1 by affecting Lrp.