Facilitated diffusion catalyzed by E-II

typhimurium normally transport trehalose via the galactose permease and are able to grow on this substrate in the complete absence of PTS phosphorylating activity. However, in S. typhimurium which lack a functional galactose permease, IfMan appears to be able to transport trehalose [78]. There is no evidence that trehalose is phosphorylated in this process, again pointing to Il -dependent transport in the facilitated diffusion mode. 

Postma and Stock [81] showed that HPr or E-I mutants were unable to grow on PTS carbohydrates suggesting that transport without phosphorylation did not take place in apparent contradiction with the studies presented above. The explanation may be that facilitated diffusion via PTS carriers is observed only in abnormal situations, carbohydrate being transported by the incorrect PTS carrier (galactose via the mannose carrier) or transport via a mutated carrier. Efflux, which also reflects facilitated diffusion, is more common for PTS carriers. 

The history of observations of efflux associated with PTS carriers is nearly as old as PTS itself. Gachelin [82] reported that A -ethylmaleimide inactivation of a-methyl-glucoside transport and phosphorylation in E. coli was accompanied by the appearance of a facilitated diffusion movement of both a-methylglucoside and glucose in both directions, uptake and efflux. His results could not discriminate, however, between one carrier operating in two different modes, active transport for the native carrier and facilitated diffusion for the alkylated carrier, or two distinct carriers. Haguenauer and Kepes [83] went on to show that alkylation of the carrier was not even necessary to achieve efflux NaF treatment which inhibits P-enolpyruvate synthesis was sufficient but this study did not address the question of one carrier or two. 

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Demonstration of catalytic activities of E-IIs other than vectorial phosphorylation, like phosphorylation of free substrate at the cytoplasmic side of the membrane and facilitated diffusion catalyzed by E-IIs and phosphorylated E-IIs. 

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