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DNA topoisomerases in Gloeothece (Nägeli) sp. ATCC 27152: a role in N 2 fixation?
Author(s) -
Page T.S.,
Gallon J. R.
Publication year - 1992
Publication title -
new phytologist
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 3.742
H-Index - 244
eISSN - 1469-8137
pISSN - 0028-646X
DOI - 10.1111/j.1469-8137.1992.tb01089.x
Subject(s) - dna supercoil , topoisomerase , dna gyrase , dna , cole1 , nitrogenase , biology , biochemistry , plasmid , microbiology and biotechnology , dna replication , escherichia coli , bacteria , gene , nitrogen fixation , genetics
summary Extracts from aerobically grown cultures of the unicellular cyanobacterium Gloeothece (Nägeli) sp. ATCC 27152 converted relaxed ColE1 plasmid cccDNA into a supercoiled form. They therefore contained a DNA gyrase. However, when DNA gyrase was inhibited, these same extracts catalyzed a net relaxation of ColE1 DNA, implying that they also contained an enzyme similar to DNA topoisomerase I. During the first 2 h after transfer of aerobically grown cultures of Gloeothece to an atmosphere of O 2 , the ability of extracts to generate supercoiled DNA was temporarily lost. Instead, extracts catalyzed net relaxation of ColE1 DNA during this period, which coincided with a transient inhibition of nitrogenase synthesis. O 2 and some related compounds also inhibited DNA supercoiling by extracts of Gloeothece in vitro. As in other diazotrophs therefore, O 2 may inhibit nitrogenase synthesis in Gloeothece by altering the relative activities of DNA gyrase and DNA topoisomerase I in such a way that relaxation of one or more of the nif genes may occur, with consequent inhibition of transcription. However, this effect is only transient. In contrast, addition of NH 4 + to cultures of Gloeothece permanently inhibited nitrogenase synthesis and did not abolish the ability of extracts to catalyze net supercoiling of DNA. Moreover, NH 4 + did not abolish DNA supercoiling in vitro. NH 4 + therefore appears to inhibit nitrogenase synthesis in Gloeothece by a mechanism different from that of O 2 .