STATIONARY-PHASE EXPRESSION OF A NOVEL ESCHERICHIA-COLI OUTER-MEMBRANE LIPOPROTEIN AND ITS RELATIONSHIP WITH MAMMALIAN APOLIPOPROTEIN-D - IMPLICATIONS FOR THE ORIGIN OF LIPOCALINS

We report a novel outer membrane lipoprotein of Escherichia coil. DNA sequencing between ampC and sugE at the 94.5 min region of the E. coil chromosome revealed an open reading frame specifying 177 amino acid r esidues. Primer extension analysis demonstrated that the promoter is a ctivated at the transition between exponential and stationary growth p hases under control of the rpoS sigma factor gene, and this was confir med in vivo by monitoring expression of beta-galactosidase activity fr om a lacZ translational fusion, The amino acid sequence exhibited 31% identity with human apolipoprotein D (apoD), which is a component of p lasma high density lipoprotein and belongs to the eukaryotic family of lipocalins, The bacterial lipocalin (Blc) contained a short deletion of 7 amino acid residues corresponding to a hydrophobic surface loop t hat is thought to facilitate the physical interaction between apoD and high density lipoprotein, However, Blc exhibited a typical prokaryoti c lipoprotein signal peptide at its amino terminus. Overexpression, me mbrane fractionation, and metabolic labeling with [H-3]palmitate demon strated that Blc is indeed a globomycin-sensitive outer membrane lipop rotein, Blc represents the first bacterial member of the family of lip ocalins and may serve a starvation response function in E. coil.