Genetic control of heterocyst formation in the blue-Green algae Nostoc muscorum and nostoc linckia

Non-heterocystous, non-nitrogenfixing (het ^- nif^-), heterocystous, non-nitrogenfixing (het ⁺ nif^-) and multiple heterocystous, nitrogen-fixing (M-het ⁺ nif⁺) mutants of heterocystous, nitrogen-fixing (het ⁺ nif⁺) wild-type Nostoc muscorum and Nostoc linckia were isolated and characterized with respect to (a) nitrogenfixing activity, (b) reversion frequency, (c) ammonium repressibility of heterocyst formation, (d) heterocyst spacing pattern, and (e) action of L-methionine-DL-sulphoximine (MSO), an inhibitor of glutamine synthetase (GS), on heterocyst regulation. The mutant and revertant results suggest: (i) either involvement of a common genetic determinant in the formation of heterocyst and nitrogenase or the organization of het genes and nif genes in a single operon prone to complete inactivation by a single polar mutation, (ii) non-participation of active nitrogenase in regulation of heterocyst spacing; (iii) involvement of genetic factor(s) in the control of heterocyst spacing pattern in N. linckia, and (iv) apparently different nature of the mechanism of heterocyst inhibition by proheterocyst from that of heterocyst inhibition by NO ₃ ^- or NH ₄ ⁺ . L-Methionine-DL-sulphoximine inhibits growth and causes heterocyst formation in chains in N. linckia growing in nitrogen-free, NO ₃ ^- , NO ₂ ^- or NH ₄ ⁺ medium, thus indicating a close physiological linkage between heterocyst and inorganic nitrogen metabolism regulation.