PURIFICATION AND PROPERTIES OF A FLAVODOXIN FROM THE HETEROCYSTOUS CYANOBACTERIUM ANABAENA-SPHAERICA

A flavodoxin was purified to homogeneity from the nitrogen-fixing hete rocystous cyanobacterium Anabaena sphaerica grown under iron-limited c onditions. The protein has a molecular mass of 21 kDa, and its spectra l properties and amino-acid composition are very close to that of flav odoxins from other cyanobacteria. A. sphaerica flavodoxin supported th e activities of A. sphaerica NADP reductase and Clostridium butyricum hydrogenase in reconstituted systems with illuminated plant chloroplas ts as reductant. With the use of polyclonal anti-flavodoxin antiserum it was found that nitrogen-fixing cultures of A. sphaerica grown under iron-sufficient conditions contain low but significant amounts of fla vodoxin (0.2-0.6 mug/mg crude extract protein) which increased dramati cally (to 8-15 mug/mg crude extract protein) after the iron concentrat ion in the medium was decreased to below 1 muM Fe. The flavodoxin cont ent of both iron-limited and iron-sufficient A. sphaerica was also sho wn to depend upon the growth phase of the (batch) cultures with a maxi mum at early exponential phase, coinciding with maximal in-vivo nitrog enase activity. These results suggest that A. sphaerica flavodoxin not only substitutes for ferredoxin under iron-limiting conditions, but a lso fulfills some specific role under iron-sufficient conditions.