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When grown in the absence of a source of combined nitrogen, the filamentous cyanobacterium Anabaena sp. strain PCC 7120 develops, within 24 h, a differentiated cell type called a heterocyst that is specifically involved in the fixation of N(2). Cell division is required for heterocyst development, suggesting that the cell cycle could control this developmental process. In this study, we investigated several key events of the cell cycle, such as cell growth, DNA synthesis, and cell division, and explored their relationships to heterocyst development. The results of analyses by flow cytometry indicated that the DNA content increased as the cell size expanded during cell growth. The DNA content of heterocysts corresponded to the subpopulation of vegetative cells that had a big cell size, presumably those at the late stages of cell growth. Consistent with these results, most proheterocysts exhibited two nucleoids, which were resolved into a single nucleoid in most mature heterocysts. The ring structure of FtsZ, a protein required for the initiation of bacterial cell division, was present predominantly in big cells and rarely in small cells. When cell division was inhibited and consequently cells became elongated, little change in DNA content was found by measurement using flow cytometry, suggesting that inhibition of cell division may block further synthesis of DNA. The overexpression of minC, which encodes an inhibitor of FtsZ polymerization, led to the inhibition of cell division, but cells expanded in spherical form to become giant cells; structures with several cells attached together in the form of a cloverleaf could be seen frequently. These results may indicate that the relative amounts of FtsZ and MinC affect not only cell division but also the placement of the cell division planes and the cell morphology. MinC overexpression blocked heterocyst differentiation, consistent with the requirement of cell division in the control of heterocyst development.

Relationship among Several Key Cell Cycle Events in the Developmental Cyanobacterium Anabaena sp. Strain PCC 7120