Organisms of three independently isolated mutants of failed to form colonies on glucose minimal agar (glucose MA) at 44°C after growth in glucose minimal salts medium at 37°C, although all three strains formed colonies on nutrient agar at 44°C. Supplementation of the glucose MA with individual amino acids including L-methionine and/or L-cysteine did not allow colony formation at 44°C, although addition of 0.1% Casamino acids was effective; replacement of glucose with other energy sources or ammonium ions with glutamate also did not allow growth at 44°C. The failure to form colonies at 44°C was not due to killing of the organisms, because colonies were formed if plates of the mutant initially incubated at 44°C were shifted to 30°C after 16 h. Introduction of the ColV, I-K94 plasmid into P678-54 , 1131 or an mutant suppressed the 44°C growth lesion, but other plasmids (F , R483ColIa, RI, ColB-K98, R124) tested in P678-54 did not. Growth of the ColV, I-K94 derivative at 44°C was due to a suppressing effect of the plasmid rather than to introduction of the plasmid into a variant with normal or altered OmpA protein. An attempt was made to ascertain which component(s) encoded by ColV, I-K94 was (were) responsible for allowing growth at 44°C. Transfer components appeared unlikely to be involved and plasmids which conferred individual colicins (plus the corresponding immunity component) did not suppress. The findings that the ColV, I-K94-encoded VmpA protein resembles the OmpA protein () immunologically and () in being a transmembrane component of the outer membrane suggested that it might be the presence of the VmpA protein which allowed growth at 44°C. Several experiments were in accord with this possibility.


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