A SHORT-FILAMENT MUTANT OF ANABAENA SP STRAIN PCC-7120 THAT FRAGMENTSIN NITROGEN-DEFICIENT MEDIUM

Strain 129 is a fragmentation mutant of the filamentous cyanobacterium Anabaena sp, strain PCC 7120, Growing with fixed nitrogen, this mutan t forms filaments that are much shorter than wild-type filaments, Foll owing starvation for fixed nitrogen, strain 129 becomes nearly unicell ular and forms few heterocysts, although electron microscopy suggests that proheterocysts form while fragmentation occurs, Starvation for su lfate, phosphate, iron, and calcium does not cause this fragmentation, The affected gene in strain 129,fraC, was cloned by complementation a nd characterized, It encodes a unique 179-amino-acid protein rich in p henylalanine, Insertional inactivation of the chromosomal copy of fraC results in a phenotype identical to that of strain 129, while complem entation using a truncated version of FraC results in only partial com plementation of the original mutant, Heterocysts could be induced to f orm in N-replete cultures of strain 129, as in wild-type cells, by sup plying extra copies of the hetR gene on a plasmid, Thus, FraC is requi red for the integrity of cell junctions in general but is apparently n ot directly involved in normal differentiation and nitrogen fixation.