Nucleotide sequence of an operon in Nostoc sp. strain ATCC 29133 encoding four genes of the oxidative pentose phosphate cycle.

Cyanobacteria are characterized as oxygenic photoautotrophs and use the reductive pentose phosphate (CalvinBenson) cycle in assimilation of C02. Based on enzyme assays and isotopic labeling experiments, a11 cyanobacteria are assumed to use reactions of the oxidative pentose phosphate pathway to generate reductant for maintenance energy in the dark period of natural diel cycles (Smith, 1983). In heterocyst-forming cyanobacteria, the two initial dehydrogenases of the oxidative pentose phosphate pathway, G6PD and 6-phosphogluconate dehydrogenase are about 70-fold more active in heterocysts than in vegetative cells and are thought to provide reductant for nitrogenase activity (Winkenbach and Wolk, 1973). zwf, the gene encoding G6PD, was isolated from a cosmid library of diazotrophic Nostoc sp. strain ATCC 29133 genomic DNA maintained in Escherichia coli (J. G. Wallis and J. C. Meeks, unpublished data). The cosmid was identified using as a heterologous probe a 500-bp interna1 fragment of zwf from the unicellular cyanobacterium Synechococcus sp. strain PCC 7942 (Table I). The 500-bp fragment was the product of PCR amplification of Synechococcus 7942 genomic DNA using degenerate primers based on conserved sequences in the zwf gene from E. coli and Zymomonas mobilis. Southern hybridization with the 500-bp fragment yielded restriction fragments of the cosmid that were identical to those of genomic DNA of Nostoc 29133. The derived amino acid sequence of Nostoc 29133 G6PD has 81% and 61% similarity to the homologous proteins from Synechococcus 7942 (Scanlan et al., 1992) and E. cozi (Rowley and Wolf, 1991), respectively. The provisional identity of zwf from sequence homology was supported by G6PD activity assays of plasmid subclones maintained in Nostoc 29133, where specific activity increased in parallel with plasmid copy number, and by lack of catalytic activity in a Table I . Cbaracteristics of a 6672-bp region from Nostoc sp. strain American Jype Culture Collection 29133 containing four genes encoding pentose phosphate cycle enzymes