The synechococcal metallothionein locus smt consists of two divergent genes: smtA coding for the metallothionein SmtA, and smtB coding for the trans-acting regulator SmtB. The latter binds at two inverted repeats, designated @entity4120 and S3/S4, in the overlapping promoter/operator sites between the two genes. We have determined the binding stoichiometries to the entire operator/promoter DNA and to the separate @entity4120 and S3/S4 half-operator oligonucleotides using sedimentation equilibrium and sedimentation velocity measurements. The full promoter/operator DNA binds two SmtB dimers. The hydrodynamic behavior of this complex supports a compact nucleoprotein structure. Each separate @entity4120 and S3/S4 operator sequence also binds two dimers. An equal molar mixture of separate @entity4120 and S3/S4 operator sequences, in excess SmtB, forms a @entity4120 -SmtB:SmtB-S3/S4 bridge complex. Combining these results with previously published binding interference data, which showed consecutive @entity4120 and S3/S4 SmtB occupancy on the operator/promoter DNA, we have developed a model for the establishment of the repression complex that appears to involve significant DNA compaction, presumably DNA bending, stabilized by SmtB-SmtB bridge interactions. DNase I footprinting titrations also showed consecutive @entity4120 and S3/S4 SmtB occupancy. The footprints expand considerably in the presence of @entity8862 . Hence, SmtB remains bound to the operator sites when @entity8862 ions are present. This result is further supported by gel retardation assay. Failure of the metal ions to dissociate SmtB from the DNA points to a hitherto unknown function of SmtB in the regulation of the smt locus.

SmtB-DNA and protein-protein interactions in the formation of the cyanobacterial metallothionein repression complex: XXXX does not dissociate the protein-DNA complex in vitro.

@entity8862 :: ('CHEBI:37256', 'Chemical') :: ['Zn2+'] @entity4120 :: ('5707;6187', 'Gene') :: ['S1/S2']
@entity8862 :: (CHEBI:37256,Chemical) :: ['Zn2+']
