Andrey Matveyev, Kathryn T. Young,* and Jeff Elhai
Dept. of Biology, University of Richmond, Richmond VA 23173

Under conditions of starvation for fixed nitrogen, Anabaena forms heterocysts capable of nitrogen fixation. The mechanism behind the spatial pattern of separation of heterocysts along a filament of Anabaena is not yet completely understood. It has been proposed that a cell's competence to differentiate is dictated by its position in the cell cycle. Due to the involvement of orphan methylases in cell cycle and differentiation in other prokaryotes, the potential role of orphan methylases in heterocyst formation in Anabaena PCC 7120 has been investigated. DmnB is an orphan methylase in Anabaena with the recognition sequence GGCC. The presence of dmnB on a multicopy plasmid in Anabaena causes formation of heterocysts in the presence of fixed nitrogen. One insertional mutant of dmnB is completely incapable of forming heterocysts even when starved of fixed nitrogen. However, wildtype heterocyst formation is not restored by the introduction of dmnB on a multicopy plasmid. Another insertional mutant of dmnB, with the inserted cassette in the reverse orientation, gives equivocal results regarding heterocyst formation. These observations leave open both the possibility that dmnB plays a fundamental role in heterocyst formation or that another gene downstream of dmnB affects heterocyst formation.