Contiguous NIF Gene Organization in a Heterocystous Cyanobacterium

Abstract

Cyanobacteria are a highly diverse, widely distributed group of photosynthetic oxygen-evolving prokaryotes that include some species capable of nitrogen fixation. The diazotrophic members of this assemblage fall into three main categories: the filamentous cyanobacteria, which possess characteristically differentiated nitrogen fixing cells called heterocysts, the filamentous cyanobacteria which lack heterocysts, and the unicellular cyanobacteria. The molecular organization of the nitrogen fixation (nif) genes in these organisms is not yet fully understood. The few reports in the literature of cyanobacteria nif gene arrangement reveal a diversity in the organization of the three nif structural genes, nif K, D and H. (1,2,3,4). Nif K and D code for nitrogenase and nif H codes for nitrogenase reductase. While some cyanobacteria, such as Synechococcus (PCC 7425), have all three genes linked (2), the nitrogenase structural genes of Anabaena sp (PCC 7120) have an 11 kilobase (kb) sequence separating nif K from the contiguous nif DH (1). The removal of this 11 kb sequence and the formation of a contiguous arrangement of nif KDH has been implicated in heterocyst differentiation and nitrogenase expression (5). This study utilized cloned fragments of the Klebsiella and Anabaena 7120 nif structural genes to determine whether or not these two arrangements are found in other cyanobacteria.