DNA topoisomerases in Gloeothece (Nägeli) sp. ATCC 27152: a role in N₂ fixation?

Abstract

SUMMARYExtracts from aerobically grown cultures of tbe unicellular cyanobacterium Gloeothece (Nageli) sp. ATCC 27152 converted relaxed ColEl plasmid cccDNA into a supercoiled form. Tbey tberefore contained a DNA gyrase. However, wben DNA gyrase was inhibited, tbese same extracts catalyzed a net relaxation of ColEl DNA, implying tbat tbey also contained an enzyme similar to DNA topoisomerase I. During tbe first 2 b after transfer of aerobically grown cultures of Gloeothece to an atmospbere of Oj, tbe ability of extracts to generate supercoiled DNA was temporarily lost. Instead, extracts catalyzed net relaxation of ColEl DNA during tbis period, wbicb coincided witb a transient inbibition of nitrogenase syntbesis. O^ and some related compounds also inbibited DNA supercoiling by extracts of Gloeothece in vitro. As in otber diazotropbs tberefore, O^ may inbibit nitrogenase syntbesis in Gloeothece by altering tbe relative activities of DNA gyrase and DNA topoisomerase I in sucb a way tbat relaxation of one or more of tbe nif genes may occur, witb consequent inbibition of transcription. However, tbis effect is only transient. In contrast, addition of NH/ to cultures of Gloeothece permanently inbibited nitrogenase syntbesis and did not abolisb tbe ability of extracts to catalyze net supercoiling of DNA. Moreover, NH^* did not abolisb DNA supercoiling in vitro. NH^* tberefore appears to inbibit nitrogenase syntbesis in Gloeothece by a mecbanism different from tbat of O^.

Diel cycling of DNA staining and nifH gene regulation in the unicellular cyanobacterium Crocosphaera watsonii strain WH 8501 (Cyanophyta)

“…The transcription of genes involved in nitrogen fixation has been shown to be sensitive to the supercoiling state of DNA in several different bacteria including Rhodopseudomonas capsulata, Klebsiella pneumoniae, Enterobacter cloacae, Sinorhizobium meliloti and the cyanobacterium Gloeothece (Nageli) sp. ATCC 27152 (Kranz and Haselkorn, 1986;Dimri and Das, 1988;Page and Gallon, 1992;Hu et al, 2000;Liu et al, 2005). Supercoiling of DNA is a topological change that can be assayed by changes in fluorescence using fluorescent DNA stains (Lebaron and Joux, 1994;Prosperi et al, 1994).…”