ISOLATION AND CHARACTERIZATION OF 2 DIFFERENT FLAVODOXINS FROM THE EUKARYOTE CHLORELLA-FUSCA

Two different molecular forms of flavodoxin from the green alga Chlore lla fusca have been purified to homogeneity and their properties compa red. The molecular masses are 22 kDa (flavodoxin I) and 20 kDa (flavod oxin II). Western blots of axenic crude extract show the two bands. Bo th are single polypeptide chains and their N-terminal sequences differ but are very similar. Each form contains 1 mol of FMN/mol of apoprote in, exhibits a typical flavodoxin u.v.-visible absorption spectrum and does not contain covalently bound phosphate. The oxidation-reduction properties of the FMN in the flavodoxins differ considerably. Redox po tentials of flavodoxin I at pH 8 are -240 mV for the oxidized/semiquin one couple and -350 mV for the semiquinone/hydroquinone couple. Flavod oxin I gives more electronegative values: -278 mV and -458 mV respecti vely. Flavodoxin II fulfils better the redox requirements for photosyn thetic electron transport and, as expected, it is more efficient at me diating NADP(+) photoreduction in the photosynthetic electron flow. A new h.p.l.c. method for flavodoxin purification is described, which is useful for the isolation of very similar anionic proteins.