TRANSFER OF A GENETIC-MARKER FROM A MEGAPLASMID OF ANABAENA SP STRAINPCC-7120 TO A MEGAPLASMID OF A DIFFERENT ANABAENA STRAIN

The 410-kb alpha megaplasmid of the heterocyst-forming cyanobacterium Anabaena sp. strain PCC 7120 was found to bear the nucA gene that enco des a sugar-nonspecific nuclease. That gene was mutated by insertion o f a cassette that confers resistance to neomycin. The resulting strain , AMP2, was mated with a streptomycin-resistant derivative of Anabaena sp. strain PCC 7118, a strain that does not form heterocysts. Cells r esistant to both neomycin and streptomycin that were derived from such matings were found to bear the neomycin resistance cassette of the do nor strain in a larger megaplasmid characteristic of the recipient str ain and did not form heterocysts. This is the first example of transfe r of a genetic marker directly between strains of cyanobacteria in whi ch incontrovertible physical evidence of transfer has been obtained. D NA sequences homologous to the nucA gene were present in 13 heterocyst -forming cyanobacteria that were tested but in none of six diverse uni cellular strains that were examined.