The capsular K5 polysaccharide of Escherichia coli is the receptor of the capsule-specific coliphage K5, which harbors an enzyme that degrades the capsular K5 polysaccharide to a number of oligosaccharides. Analysis of the degradation products using gel permeation chromatography, the periodate-thiobarbituric acid and bicinchoninic acid reactions, and nuclear magnetic resonance spectroscopy showed that the major reaction products are hexa-, octa-, and decasaccharides with 4,5-unsaturated glucuronic acid (⌬ 4,5 GlcA) at their nonreducing end. Thus, the bacteriophage enzyme is a K5 polysaccharide lyase and not, as we had reported previously, an endo-N-acetylglucosaminidase.

Expression of the Escherichia coli K5 antigen was used as a model system to study the role of known regulators of gene expression on production of group II capsules in E. coli. Only mutations in the rfaH gene had an effect on production of the K5 antigen, abolishing the expression of any detectable capsule at 37 degrees C. None of the mutations studied induced capsule expression at 18 degrees C. A sequence, termed JUMPstart, found in group II capsule gene clusters and upstream of a number of polysaccharide biosynthesis genes in enteric bacteria is homologous to sequences found in RfaH regulated operons. This may indicate a common mode of regulation of these polysaccharide biosynthesis genes by RfaH.