GROWTH OF THE CYANOBACTERIUM ANABAENA ON MOLECULAR NITROGEN - NIFJ IS REQUIRED WHEN IRON IS LIMITED

The nifJ gene of Klebsiella pneumoniae encodes an oxidoreductase required for the transfer of electrons from pyruvate to flavodoxin, which reduces nitrogenase. The nifJ gene of Anabaena 7120, isolated from a cosmid bank, was found to contain an open reading frame encoding a 1197-aa protein. The deduced amino acid sequence shows 50% identity to the Klebsiella homolog. The nifJ gene in Anabaena 7120 was inactivated by chromosomal interruption. The resulting mutant was unable to grow on medium depleted of both iron and combined nitrogen but grew normally, fixing nitrogen, when iron was present. NifJ transcripts of 2.7 and 4.3 kb are induced by iron depletion irrespective of nitrogen status. One particular stretch of the Anabaena 7120 nifJ gene encodes 12 aa with no complementary matches in the Klebsiella protein. This insert contains five tandem repeats of the heptamer CCCCAGT. These heptamers, as well as heptamers and octamers of other related sequences, have been located in a number of cyanobacterial genomes but are usually not found within the coding region of a gene. The site of the Anabaena 7120 heptamers in the nifJ genes of other filamentous cyanobacteria contains a surprising diversity of repeated sequences, both octamers and heptamers. The corresponding protein inserts range in length from 1 to 21 aa, relative to Klebsiella NifJ.