Genetic analysis of the het and nif genes in the blue-green alga Nostoc muscorum

Summary

Two multiply marked complementary strains namely Het ⁺ Nif⁺ Str-R and Het ^- Nif^- Ery-R MSO-R were constructed and crossed under conditions counterselective for the Het ⁺ Nif⁺ Str-R parent and selective only for recombinants of Str-R and Ery-R or Str-R and MSO-R constitution. The results of the recombinant analysis with regard to the selected and unselected markers suggested that the Het ^- Nif^- Ery-R MSO-R parent acted as a recipient and the Het ⁺ Nif⁺ Str-R parent as donor of the genetic markers in the cross. The joint inheritance of Het ⁺ and Nif ⁺ unselected markers among the recombinants was found to occur more frequently than the inheritance of the Het ⁺ or Nif ⁺ markers alone. The observed joint inheritance of Het ⁺ and Nif ⁺ markers among the recombinants probably results from the inheritance of the regulatory gene(s) required for the activation of latent het and nif genes. This interpretation is fully supported by (a) the frequency distribution of unselected Het ⁺ and Nif ⁺ markers and (b) the reversion frequency of Het ^- Nif ^- strains to Het ⁺ Nif⁺ prototrophy. Accordingly the apparent close genetic linkage of het and nif genes is not due to their organization in a single operon but to their common regulation by regulatory gene(s) of a positive control nature. The Het ⁺ Nif⁺ wild type, mutant, revertant, and recombinant strains all appear similar in their NO ^-₃ repression of both heterocyst and nitrogenase. The Het ⁺ Nif^- and Het ^- Nif⁺ recominants also show similar NO ^-₃ repression of their heterocyst and nitrogenase respectively. The presence of only microaerobic acetylene reducing activity in Het ^- Nif⁺ recombinants clearly indicates the heterocyst to be an organ for protection of nitrogenase against oxygen toxicity.