SUMMARY: The outer membrane (OM) proteins of a pv. strain (HS191), capable of causing holcus leaf spot of corn and other grasses, and two plasmidless derivatives of HS191, one avirulent (AO111) and one with reduced virulence (PSG100), were isolated by selective solubilization of the cytoplasmic membrane (CM) with 0.4% (w/v) SDS or by centrifugation on sucrose density gradients. OM preparations were enriched in hexosamine and 2-keto-3-deoxyoctonate and contained little of the CM enzymes NADH oxidase and malate dehydrogenase. Characterization of OM preparations by SDS-PAGE and isoelectric focusing indicated a minimum of 20 OM proteins; protein bands 8 and 9 quantitatively predominated. In addition, some of the proteins were modified by heat and others by 2-mercaptoethanol. Although protein 5 was absent in both plasmidless strains, AO111 differed from PSG100 in lacking protein 5a and having two proteins (8b’ and 8c’) with different pi values. It is suggested that plasmid pCG131 coded for protein 5 (mol. wt 68 000) and repressed the synthesis of proteins 4a (78000) and 5a (63000). The relationship of specific changes in OM protein composition to virulence is discussed.


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