PURIFICATION AND CHARACTERIZATION OF A SYNECHOCOCCUS SP STRAIN PCC 7942 POLYPEPTIDE STRUCTURALLY SIMILAR TO THE STRESS-INDUCED DPS PEXB PROTEIN OF ESCHERICHIA-COLI/

A stable DNA/protein complex having an apparent molecular mass of appr oximately 150 kDa was purified from nitrate-limited cultures of the cy anobacterium Synechococcus sp. strain PCC 7942. Amino-terminal peptide sequencing indicated that the polypeptide was structurally similar to the Dps protein of Escherichia coli; Dps is also known as the product of the starvation- and stationary-phase-inducible gene, pexB. The 150 -kDa complex dissociated into a 22-kDa protein monomer after boiling i n 2% SDS. The 150-kDa complex preparation had approximately a 10% nucl eic acid content and upon dissociation released DNA fragments that wer e sensitive to S1 nuclease digestion. Immunoblot data indicated that t he complex accumulates during stationary phase and during nitrogen, su lfur, and phosphorus limitation. DNA-binding assays indicated that the protein nonspecifically binds both linear and supercoiled DNA. Circul ar dichroism spectroscopy revealed that the Synechococcus sp. Dps-like protein contains extensive regions of alpha-helical secondary structu re. We propose that the 150-kDa complex represents a hexameric aggrega te of the Dps-like protein complexed with single-stranded DNA and serv es to bind a portion of the chromosomal DNA under nutrient-limited con ditions.