ZN2-SENSING BY THE CYANOBACTERIAL METALLOTHIONEIN REPRESSOR SMTB - DIFFERENT MOTIFS MEDIATE METAL-INDUCED PROTEIN-DNA DISSOCIATION()

SmtB is a member of a family of repressors which dissociate from DNA i n the presence of metals; Zn2+ being the most potent inducer of metall othionein gene (smtA) transcription in vivo, In Synechococcus PCC 7942 cells devoid of chromosomal smtB, four plasmid-encoded mutants of Smt B (C61S, T11S/C14S, C121S and H105R/H106R) repressed lacZ expression d riven by the smtA operator-promoter. Gel retardation assays with extra cts from the complemented cells detected multiple SmtB-dependent compl exes similar to those obtained with extracts from wild-type cells or w ith recombinant-SmtB. Elevated [Zn2+] alleviated repression in vivo by all of the mutants except H105R/H106R. These His residues (one or bot h) are therefore essential for Zn2+-sensing while, contrary to expecta tions, Cys residues are not, Hence different motifs facilitate metal-i nduced DNA-dissociation by SmtB and ArsR (the related oxyanion-sensing repressor), presumably generating variety in the spectra of metals se nsed, Nucleotides and amino acids involved in DNA-SmtB interaction hav e been further defined/inferred and we also confirm that additional un known factors form specific associations with the smt operator-promote r in elevated [Zn2+].