EXPRESSION AND MUTAGENESIS OF THE DPSA GENE OF SYNECHOCOCCUS SP. PCC7942, ENCODING A DNA-BINDING PROTEIN INVOLVED IN OXIDATIVE STRESS PROTECTION

The Dps family of proteins are a diverse group of bacterial stress-ind ucible polypeptides that bind DNA and likely confer resistance to pero xide damage during periods of oxidative stress and long-term nutrient limitation, Some members of the Dps protein family have been shown to form abundant, large (similar to 150 kD) hexameric complexes that bind chromosomal DNA with little sequence specificity. Previous work from this lab has demonstrated that the Dps proteins are divergent members of the bacterioferritin/bacterioferritin superfamily, and that the Syn echococcus sp. PCC7942 Dps homolog, named DpsA, is a DNA-binding hemop rotein having heme-dependent catalytic activity. We speculated that th is protein may yield a peroxide-consuming mechanism located on the chr omosomal DNA, and we also suggested that this activity may be a necess ary feature ro handle the endogenous oxidative stresses associated wit h oxygenic photosynthesis. Current work has examined the expression of dpsA both under nutrient stress and during the growth phase; whereas dpsA mRNA is detectable in the exponential phase, transition to statio nary phase yields a 20-fold increase in steady-state mRNA levels, Mapp ing the promoter region identifies a TAGAAT -10 sequence likely recogn ized by a cyanobacterial RpoS homolog. Lastly, site-directed mutants l acking dpsA function exhibit a severe phenotype impaired under all con ditions yielding photooxidative stress; these include high light and t reatment with paraquat. This supports our contention that the DpsA pro tein serves an important protective function in an obligate photoautot roph.