SUMMARY: Addition of the amino acid analogue -7-azatryptophan (AT) to dinitrogen-fixing cultures of the cyanobacterium , and to cultures in which heterocyst development was induced by the removal of fixed nitrogen from the medium, resulted in the development of many adjacent (double) heterocysts. Cell division in was asymmetrical, with a maximum of 10% of all cell divisions producing two daughter cells of equal size. During incubation with AT the frequency of symmetrical cell divisions remained unchanged, indicating that the preponderance of double heterocysts induced by the analogue did not result from any change in the symmetry of cell division. Incubation of cultures with AT resulted in a decrease in the number of vegetative cells between heterocysts (the interheterocyst interval). The extent of the decrease was proportional to the length of the incubation period in the presence of AT. Double heterocysts, which constitute a zero interval, developed at a time when the minimum interval was three cells in dinitrogen-fixing cultures, or nine cells in cultures induced to differentiate by the removal of fixed nitrogen from the medium. These observations have been used to formulate a model to explain the influence of AT on the control of heterocyst development and spacing. In this model the inactive form of an inhibitor of heterocyst development is produced constitutively by vegetative cells and is activated either by a co-inhibitor derived from developing or mature heterocysts, or by high concentrations of fixed nitrogen.


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