The facultatively chemolithoautotrophic hydrogen-oxidizing bacteria Alcaligenes eutrophus and Alcaligenes hydrogenophilus partially derepressed the formation of phosphoribulokinase and ribulosebisphosphate carboxylase during heterotrophic growth on fructose or gluconate. We examined whether the indigenous magaplasmids in these bacteria that encode the ability to oxidize hydrogen affected this derepression. The results suggest an involvement of the plasmids in the derepression for the following reasons:
(i) wild-type strains, except A. eutrophus TF 93, exhibited the derepressible phenotype; (ii) plasmid-cured mutants formed the enzymes with formate as autotrophic growth substrate but did not derepress their formation during heterotrophic growth; (iii) the phenotype of the wild type was restored by transfer of the plasmids into plasmid-cured mutants. Plasmid priG2 from strain TF93 differed from the other wild-type plasmids by conferring a non-derepressible phenotype onto the harboring strain. Mutants of A. eutrophus H16 carrying deletions in plasmid priG1 showed a similar phenotype as that of the plasmid-cured mutants. We concluded that the plasmids from the various strains studied encode a regulatory ability to derepress phosphoribulokinase and ribulosebisphosphate carboxylase under heterotrophic growth conditions.