Variant forms of matrix protein in Escherichia coli B/r bearing N plasmids.

Plasmids of the N incompatibility group have been found to decrease or virtually eliminate the synthesis of the 36,500 dalton outer membrane matrix protein of their Escherichia coli B/r hosts (Iyer, R. (1977) Biochim. Biophys. Acta 470, 258--272 and Iyer, R., Darby, V. and Holland, I.B. (1978) FEBS Lett. 85, 127--132) or modify its composition. Although the 34,000 dalton tol G protein is slightly increased in some strains, it is identical in composition to the homologous protein from the plasmidless host. In three of five N+ strains the synthesis of the modified matrix proteins depends on the temperature of cultivation of the strains in which they occur. The alterations to the matrix proteins are non-identical and do not affect the expression of several plasmid-coded functions including those of sensitivity to the N plasmid-specific filamentous bacteriophage IKe (Khatoon, H. and Iyer, R. (1971) Can. J. Microbiol. 17, 669--675), or their interbacterial transfer via conjugation to appropriate recipient strains. Thus, although the significance of the variant matrix proteins in N+ strains with respect to plasmid-mediated functions remains unclear, N plasmids nevertheless provide a convenient system which might be used to elucidate the events that precede the insertion of this protein into the outer membrane of E. coli B/r hosts.