ELEMENTS INTERRUPTING NITROGEN-FIXATION GENES IN CYANOBACTERIA - PRESENCE AND ABSENCE OF A NIFD ELEMENT IN CLONES OF NOSTOC SP STRAIN MAC

Nostoc sp. strain Mac is capable of microaerobic, but not aerobic, nit rogen fixation (Fox(-)). Nostoc Mac grows as long, relatively straight , filaments that are well dispersed in the culture medium. However, sp ontaneously-arising revertant strains selected for aerobic nitrogen fi xation (Fox(+)) all grow as coiled filaments that associate in macrosc opic clumps or balls of varying dimensions, DNA restriction fragment l ength polymorphism, using nitrogenase (nif) structural genes as probes , established identity between revertants and the parental culture. Ma pping of the fragments and lack of hybridization to specific probes in dicated the absence of a DNA sequence interrupting the nifD gene in on e Fox(+) revertant. Such a nifD element is assumed to be present in es sentially all heterocyst-forming cyanobacteria. Only one clone out of 223 Fox(-) and Fox(+) Nostoc Mac clones surveyed lacked the nifD eleme nt, indicating that loss of the element is a rare event, The nifD elem ent is present in the same location in the genome of Nostoc Mac as it is in all other heterocyst-forming cyanobacteria analysed. No phenotyp ic differences could be detected between two Fox(+) clones containing or lacking the nifD element, including repression and derepression of nitrogen fixation in response to the presence or absence of combined n itrogen. We suspect that retention of the nifD element in vegetative c ells of heterocyst-forming cyanobacteria is a consequence of selective pressure, although such selective conditions in laboratory cultures h ave not been identified.