Comparative Analysis Of Heavy-Metal Ion Sensing Mechanisms With Transcription Factors, Smtbs, From Freshwater Synechococcus Sp. PCC 7942, And Marine Synechococcus Sp. PCC 7002: Evolutionary And Structural Aspects

Metal-responsive transcription factors play a crucial role in the metal ion homeostasis with both cellular metabolism and environmental metal availability. In present work, we studied structural and evolutionary features of a transcription repressor, SmtB from freshwater Synechococcus sp. PCC 7942 and its homologs from other cyanobacteria in detail. We mined putative SmtB-like zinc sensors from all available sequenced genomes of cyanobacterial species in CyanoBase database. We found marine Synechococcus sp. PCC 7002 consists of similar homologous gene regulation system to that of PCC 7942. In both cyanobacteria, the gene expression of SmtA (metallothionein) is regulated by repressor protein SmtB and they are transcribed divergently. We compared the binding affinities of SmtBs to the operator/promoter regions of smtA and inhibitory effect of Zn 2+ with electrophoretic mobility shift assay. From these analyses, we found number of complexes between recognition DNA sequences and repressor proteins depends on the structural aspects of DNA sequences of operator/promoter region. We also found that the α5 sites are completely identical in these two proteins, but differences in α3N sites and/or operator/promoter region might be responsible for dissociation of protein-DNA complexes at different zinc ion concentrations (16 μM of Zn 2+ in PCC 7942 and 256 μM of Zn 2+ in case of PCC 7002).