SUMMARY: The differentiation of proheterocysts and heterocysts in was initiated by the removal of NHCl from the medium. The development of both proheterocysts and mature heterocysts was inhibited by rifampicin, chloramphenicol and mitomycin C, at final concentrations of 0.2, 4.0 and 1.0 μg ml, respectively. However, lower concentrations than these were sufficient to inhibit mature heterocyst development while permitting proheterocyst formation. The presence of rifampicin (0.2 μg ml) during the early stages of the induction of differentiation, just prior to the initial visual detection of proheterocysts, led to an increase in the frequency of heterocysts that subsequently developed. To assess the commitment of cells to differentiation, samples of culture were incubated with inhibitors or NHCl, or were transferred to darkness, at different times following the initiation of development. Each treatment produced a characteristic commitment time which differed for proheterocysts and mature heterocysts. The data obtained from these experiments have been used to formulate a model for the control of heterocyst development in in which DNA replication serves as a timer mechanism for the process.


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