A novel genome rearrangement involved in heterocyst differentiation of the cyanobacteriumAnabaenasp. PCC 7120

A new procedure for the preparation of intact microbial DNA allowed us to obtain DNA, suitable for pulsed-field gel electrophoresis, from both vegetative cells and heterocysts (differentiated cells with a potential for nitrogen fixation) of the cyanobacterium Anabaena PCC 7120. Through this procedure it was possible to locate genomic developmental rearrangements by visualizing the increased mobility of large heterocyst DNA fragments undergoing rearrangements. The 390-kb SalI fragment of vegetative cell DNA was shown to lose about 70 kb as a result of the previously reported 11- and 55-kb deletions, restoring functional nif operons. A new developmental rearrangement was also detected. This takes place more than 600 kb upstream of the nif operons and results in the excision of about 18 kb from the 505-kb fragment.