AN INSERTION ELEMENT PREVENTS PHYCOBILISOME SYNTHESIS IN N-2-FIXING SYNECHOCYSTIS SP STRAIN BO-8402

The unicellular diazotrophic cyanobacterium Synechocystis sp, strain B O 8402, isolated from Lake Constance, contains a novel insertion seque nce, IS8402, in the apcA gene encoding a pigmented protein of phycobil isomes. IS8402 comprises 1,322 bp, flanked by two inverted terminal re peats of 15 bp, Upon insertion in the target DNA, direct duplications of 8 nucleotides were generated, One open reading frame, potentially c oding for a protein of 399 amino acids, was found, The deduced amino a cid sequence shows homology to putative transposases of the IS4 family , Precise excision of the insertion element resulted in a spontaneous revertant, Synechocystis sp, strain BO 9201, that had regained the abi lity to form hemidiscoidal phycobilisomes. Apart from the unique inser tion of IS8402 into apcA in strain BO 8402, both strains contain at le ast 12 further homologous insertion elements at corresponding sites in the genomes, The unique insertion in strain BO 8402 prevents the expr ession of the apcABC operon and hence abolishes the formation of intac t phycobilisomes. This decreases the quantum efficiency of photosystem II and promotes anaerobic photosystem I-dependent nitrogenase activit y, The insertion may represent an initial step to optimize anaerobic N -2 fixation in a unicellular cyanobacterium with a highly oxygen-sensi tive nitrogenase.