Developmental regulation and spatial pattern of expression of the structural genes for nitrogenase in the...

Depriving the cyanobacterium Anabaena of fixed nitrogen induces the differentiation of heterocysts at intervals along its filaments. To test whether the oxygen-deficient conditions believed to prevail within mature heterocysts are sufficient, in the absence of fixed nitrogen, to elicit the expression of nitrogenase, PnifHDK was fused transcriptionally to luxAB (encoding luciferase). Expression, monitored from individual cells as light emission, was localized (with a resolution of approximately 1 micron) to differentiated cells, whether or not oxygen was present. Anabaena PCC 7118 is a heterocystless mutant strain that is known to fix nitrogen when deprived of combined nitrogen under anaerobic conditions. Three lines of evidence indicate that the mutant has retained the ability to develop a pattern despite its inability to make heterocysts. First, morphologically distinct cells appear at nonrandom intervals when filaments are starved of nitrogen. Second, these cells, like heterocysts, have little or no phycocyanin-dependent fluorescence. Third, nitrogen-starved filaments fragment, with fragment lengths similar to the spacing normally seen between heterocysts. Expression of PnifHDK-luxAB was largely confined to differentiated cells in the mutant as in the wild-type strain. These results provide evidence for a causal relationship between development and transcriptional events in Anabaena.