Group four capsule operon and GfcD.
(a) The Escherichia coli group 4 operon consists of seven genes. The genes gfcA, gfcB, gfcC, and gfcD encode proteins of an unknown function. Genes gfcE, etp, and etk share homology with proteins of Group 1 capsule export systems. Distant homologs are also found in Vibrio and Burkholderia species. (b) Membrane topology of GfcD as predicted by HHOMP and classified as a potential outer membrane β-barrel protein. The central region (273–427) is devoid of transmembrane strands and may be an independently folded domain with the predicted secondary structure shown. The signal sequence at the N-terminus (shaded blue) shows a putative signal peptidase II cleavage and acylation site (arrow 19) as previously annotated, and the peptidase I cleavage site (arrow 22) revealed in the present work by MALDI-TOF and N-terminal sequencing.