The filamentous heterocyst-forming cyanobacterium Richelia intracellularis forms associations with diatoms and is very abundant in tropical and subtropical seas. The genus Richelia contains only one species, R. intracellularis Schmidt, although it forms associations with several diatom genera and has considerable variation in size and morphology. The genetic diversity, and possible host specificity, within the genus Richelia is unknown. Using primers against hetR, a gene unique for filamentous cyanobacteria, specific polymerase chain reaction (PCR) products were obtained from natural populations of R. intracellularis filaments associated with three diatom genera. Phylogenetic analyses of these sequences showed that they were all in the same clade. This clade contained only the R. intracellularis sequences. The genetic affiliation of hetR sequences of R. intracellularis to those of other heterocystous cyanobacteria strongly suggests that it was not closely related to endosymbiotic Nostoc spp, hetR sequences. Sequences from R, intracellularis-Hemiaulus membranaceus sampled in the Atlantic and Pacific Oceans were almost identical, demonstrating that the genetic relatedness was not dependent on geographical location, All sequences displayed a deep divergence between symbionts from different genera and a high degree of host specificity.

A polymerase chain reaction-based method was used to isolate a Nostoc sp. PCC 9229 cDNA from infected glands of Gunnera chilensis. The complete gene sequence was isolated from a genomic Nostoc sp. PCC 9229 library. Sequence analysis showed 84% amino acid similarity to a putative cyclodextrin glycosyltransferase from Nostoc sp. PCC 7120 and the gene was therefore termed cgt. Southern blot revealed that the cgt gene was present in symbiotically competent cyanobacteria. The cgt gene was expressed in free-living nitrogen-fixing cultures in light or in darkness when supplemented with fructose. This is the first expression analysis of a cgt gene from a cyanobacterium. (C) 2003 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.

The cyanobacterium Nostoc PCC 9229 forms an intracellular nitrogen-fixing symbiosis with the angiosperm Gunnera. In symbiosis the cyanobacterium is enclosed in darkness and receives carbon from the plant in an unknown form. Out of five putative plant carbohydrate sources tested in vitro, fructose and glucose were found to support nitrogen fixation in darkness. The other three dextrin, sucrose and Gunnera sp. mucilage could not induce nitrogenase activity in darkness. The stimulatory effect by fructose was also observed in illuminated samples. After four weeks incubation in darkness, nitrogenase was still active in cultures when fructose was added and multiple thick-walled nitrogen-fixing cells (heterocysts) were observed, and chlorophyll levels unchanged. The expression as shown by Northern blot analysis revealed that fructose influenced the gene expression of hetR, a gene necessary for heterocyst formation, in darkness. Fructose and glucose may therefore be the carbohydrates supplied by the host plant to induce heterocyst differentiation and nitrogen fixation in the cyanobiont Nostoc PCC 9229.