NITROGEN-FIXATION AND NIF GENE ORGANIZATION IN BRANCHED HETEROCYSTOUSCYANOBACTERIA - VARIATION IN THE PRESENCE OF XISA

Five branched heterocystous cyanobacteria (Scytonematopsis sp., Scyton ema sp., Tolypothrix ceylonica, Mastigocladus sp. and Fischerella sp.) were examined for their pattern of induction of nitrogenase activity and nif gene organization. All the forms showed the onset of nitrogena se activity after 12 h which could be correlated with the appearance o f proheterocysts. The highest activity was exhibited by T. ceylonica. Hybridization studies revealed the presence of the nifD gene but the a bsence of the xisA gene in Mastigocladus sp. and Scytonematopsis sp. I nterestingly, Tolypothrix sp, and Scytonema sp. DNA samples hybridized with xisA. Hence no uniformity seems to exist regarding the presence of xisA and the related nif gene organization in branched heterocystou s cyanobacteria. This investigation throws light on the primitive char acter and phylogenetic relatedness of branched forms to the coccoid/co lonial forms. It also provides evidence for the proposition that stigo nematacean cyanobacteria may not represent the most advanced cyanobact erial forms; rather they may link the coccoid and filamentous forms.