H-060. Structural Conservation of the Lrp Regulon

P. Srivastava1, B. M. Vaz2, A. B. Khodursky1;
1Univ. of Minnesota, Saint Paul, MN, 2Hamline Univ., Saint Paul, MN.

In Escherichia coli K12 the leucine-responsive regulatory protein (Lrp) affects expression of ~400 genes. Lrp is highly conserved at the amino acid level among many gram negative bacteria. Our study checks the hypothesis that species with highly conserved Lrp orthologs show a conserved pattern of Lrp dependent regulation. EHEC causes hemolytic-uremic syndrome and has Lrp ortholog which is 100% conserved at the amino acid level. It differs from E. coli K12 in having an extra ~25% of genes that are scattered over the entire chromosome. Therefore the work was also aimed to reveal the extent to which new members (including genes relevant to virulence) have joined the regulon. To compare Lrp regulon structures in two organisms, transcript profiles under the same growth conditions were obtained using WT and lrp knock-out strains. To map direct Lrp targets in the chromosomes of both species chromatin immunoprecipitation of LRP-bound DNA sequences followed by hybridization on microarray was carried out. The Lrp regulon was operationally defined as a collection of genes whose expression is affected in an lrp mutant and whose regulatory sequences are bound by the Lrp protein, both defined with at least 95% confidence. The regulons were compared between the two species. It was observed that the EHEC regulon fully encompasses the K12 one. In EHEC, 29 “new” species-specific genes, including some in the LEE locus, could be assigned to the Lrp regulon. Given the proportion of direct Lrp targets in E. coli genome (0.03) and the number of EHEC accrued genes, one would expect at least 49 new Lrp targets in EHEC. The discrepancy (p-value <10-3) could be attributed to multiple instrumental and/or biological factors. Thus we conclude that the structure of the Lrp regulon is highly conserved among the two bacterial species and the Lrp regulon in E. coli O157:H7 has maintained the basic membership and regulatory patterns of the K12 version.