Certain filamentous nitrogen-fixing cyanobacteria generate signals that direct their own multicellular development. They also respond to signals from plants that initiate or modulate differentiation, leading to the establishment of a symbiotic association. An objective of this review is to describe the mechanisms by which free-living cyanobacteria regulate their development and then to consider how plants may exploit cyanobacterial physiology to achieve stable symbioses. Cyanobacteria that are capable of forming plant symbioses can differentiate into motile filaments called hormogonia and into specialized nitrogen-fixing cells called heterocysts. Plant signals exert both positive and negative regulatory control on hormogonium differentiation. Heterocyst differentiation is a highly regulated process, resulting in a regularly spaced pattern of heterocysts in the filament. The evidence is most consistent with the pattern arising in two stages. First, nitrogen limitation triggers a nonrandomly spaced cluster of cells (perhaps at a critical stage of their cell cycle) to initiate differentiation. Interactions between an inhibitory peptide exported by the differentiating cells and an activator protein within them causes one cell within each cluster to fully differentiate, yielding a single mature heterocyst. In symbiosis with plants, heterocyst frequencies are increased 3- to 10-fold because, we propose, either differentation is initiated at an increased number of sites or resolution of differentiating clusters is incomplete. The physiology of symbiotically associated cyanobacteria raises the prospect that heterocyst differentiation proceeds independently of the nitrogen status of a cell and depends instead on signals produced by the plant partner

Hornworts comprise a bryophyte lineage that diverged from other extant land plants >400 million years ago and bears unique biological features, including a distinct sporophyte architecture, cyanobacterial symbiosis and a pyrenoid-based carbonconcentrating mechanism (CCM). Here, we provide three high-quality genomes of Anthoceros hornworts. Phylogenomic analyses place hornworts as a sister clade to liverworts plus mosses with high support. The Anthoceros genomes lack repeat-dense centromeres as well as whole-genome duplication, and contain a limited transcription factor repertoire. Several genes involved in angiosperm meristem and stomatal function are conserved in Anthoceros and upregulated during sporophyte development, suggesting possible homologies at the genetic level. We identified candidate genes involved in cyanobacterial symbiosis and found that LCIB, a Chlamydomonas CCM gene, is present in hornworts but absent in other plant lineages, implying a possible conserved role in CCM function. We anticipate that these hornwort genomes will serve as essential references for future hornwort research and comparative studies across land plants.

Relative deficiency of pentraxin proteins is implicated in the pathogenesis of systemic lupus erythematosus. The C-reactive protein (CRP) response is defective in patients with acute flares of disease, and mice with targeted deletions of the serum amyloid P component gene (Sap) develop a lupus-like illness. In humans, the genes for CRP (CRP) and SAP (APCS) map to 1q23.2 within an interval linked with SLE. We have investigated the candidate genes CRP and APCS in two cohorts totalling 586 UK simplex SLE families. The inheritance of an intronic dinucleotide repeat and seven single nucleotide polymorphisms in the CRP and APCS genes was examined by application of family-based tests of association and linkage. Basal levels of CRP were influenced independently by two polymorphisms at the CRP locus, CRP 2 and CRP 4. Furthermore, the latter polymorphism was linked/associated with SLE and antinuclear autoantibody production. Thus, the polymorphism associated with reduced basal CRP was also associated with the development of SLE. These data support the hypothesis that defective disposal of potentially immunogenic material is a contributory factor in lupus pathogenesis. The identification of polymorphisms that determine basal CRP levels has implications in ischaemic heart disease, where CRP level is an important predictor of risk.

Heterocysts, sites of nitrogen fixation in certain filamentous cyanobacteria, are limited to a heterotrophic metabolism, rather than the photoautotrophic metabolism characteristic of cyanobacterial vegetative cells. The metabolic route of carbon catabolism in the supply of reductant to nitrogenase and for respiratory electron transport in heterocysts is unresolved. The gene (zwf) encoding glucose-6-phosphate dehydrogenase (G6PD), the initial enzyme of the oxidative pentose phosphate pathway, was inactivated in the heterocyst-forming, facultatively heterotrophic cyanobacterium, Nostoc sp. strain ATCC 29133. The zwf mutant strain had less than 5% of the wild-type apparent G6PD activity, while retaining wild-type rates of photoautotrophic growth with NH 4 ؉ and of dark O 2 uptake, but it failed to grow either under N 2 -fixing conditions or in the dark with organic carbon sources. A wild-type copy of zwf in trans in the zwf mutant strain restored only 25% of the G6PD specific activity, but the defective N 2 fixation and dark growth phenotypes were nearly completely complemented. Transcript analysis established that zwf is in an operon also containing genes encoding two other enzymes of the oxidative pentose phosphate cycle, fructose-1,6-bisphosphatase and transaldolase, as well as a previously undescribed gene (designated opcA) that is cotranscribed with zwf. Inactivation of opcA yielded a growth phenotype identical to that of the zwf mutant, including a 98% decrease, relative to the wild type, in apparent G6PD specific activity. The growth phenotype and lesion in G6PD activity in the opcA mutant were complemented in trans with a wild-type copy of opcA. In addition, placement in trans of a multicopy plasmid containing the wild-type copies of both zwf and opcA in the zwf mutant resulted in an approximately 20-fold stimulation of G6PD activity, relative to the wild type, complete restoration of nitrogenase activity, and a slight stimulation of N 2 -dependent photoautotrophic growth and fructose-supported dark growth. These results unequivocally establish that G6PD, and most likely the oxidative pentose phosphate pathway, represents the essential catabolic route for providing reductant for nitrogen fixation and respiration in differentiated heterocysts and for dark growth of vegetative cells. Moreover, the opcA gene product is involved by an as yet unknown mechanism in G6PD synthesis or catalytic activity.Cyanobacteria are a morphologically diverse but phylogenetically cohesive group of gram-negative eubacteria that are phenotypically united by their oxygen-evolving photosynthetic mechanism. All cyanobacteria are photoautotrophic, using ATP and NADPH generated in the light reactions for autotrophic CO 2 assimilation by enzymes of the Calvin-Benson, or reductive pentose phosphate, cycle. They synthesize glycogen as the storage material in the light, and this glycogen is subsequently catabolized to provide maintenance energy during the dark periods of natural day-night cycles.The vegetative cells of many filamentous cya...

The vegetative cells of the filamentous cyanobacterium Nostoc punctiforme can differentiate into three mutually exclusive cell types: nitrogen-fixing heterocysts, spore-like akinetes, and motile hormogomium filaments. A DNA microarray consisting of 6,893 N. punctiforme genes was used to identify the global transcription patterns at single time points in the three developmental states, compared to those in ammonium-grown time zero cultures. Analysis of ammonium-grown cultures yielded a transcriptome of 2,935 genes, which is nearly twice the size of a soluble proteome. The NH 4 ؉ -grown transcriptome was enriched in genes encoding core metabolic functions. A steady-state N 2 -grown (heterocyst-containing) culture showed differential transcription of 495 genes, 373 of which were up-regulated. The majority of the up-regulated genes were predicted from studies of heterocyst differentiation and N 2 fixation; other genes are candidates for more detailed genetic analysis. Three days into the developmental process, akinetes showed a similar number of differentially expressed genes (497 genes), which were equally up-and down-regulated. The down-regulated genes were enriched in core metabolic functions, consistent with entry into a nongrowth state. There were relatively few adaptive genes up-regulated in 3-day akinetes, and there was little overlap with putative heterocyst developmental genes. There were 1,827 differentially transcribed genes in 24-h hormogonia, which was nearly fivefold greater than the number in akinete-forming or N 2 -fixing cultures. The majority of the up-regulated adaptive genes were genes encoding proteins for signal transduction and transcriptional regulation, which is characteristic of a motile filament that is poised to sense and respond to the environment. The greatest fraction of the 883 down-regulated genes was involved in core metabolism, also consistent with entry into a nongrowth state. The differentiation of heterocysts (steady state, N 2 grown), akinetes, and hormogonia appears to involve the up-regulation of genes distinct for each state.

SummaryIn filamentous cyanobacteria, the mechanism of gliding motility is undefined but posited to be driven by a polysaccharide secretion system known as the junctional pore complex (JPC). Recent evidence implies that the JPC is a modified type IV pilus-like structure encoded for in part by genes in the hps locus. To test this hypothesis, we conducted genetic, cytological and comparative genomics studies on hps and pil genes in Nostoc punctiforme, a species in which motility is restricted to transiently differentiated filaments called hormogonia. Inactivation of most hps and pil genes abolished motility and abolished or drastically reduced secretion of hormogonium polysaccharide, and the subcellular localization of several Pil proteins in motile hormogonia corresponds to the site of the junctional pore complex. The non-motile ΔhpsE-G strain, which lacks three glycosyltransferases that synthesize hormogonium polysaccharide, could be complemented to motility by the addition of medium conditioned by wild-type hormogonia. Based on this result, we speculate that secretion of hormogonium polysaccharide facilitates but does not provide the motive force for gliding. Both the Hps and Pil homologs characterized in this study are almost universally conserved among filamentous cyanobacteria, with the Hps homologs rarely found in unicellular strains. These results support the theory that Hps and Pil proteins compose the JPC, a type IV pilus-like nanomotor that drives motility and polysaccharide secretion in filamentous cyanobacteria.

No abstract

Nostoc punctiforme is a phenotypically complex, filamentous, nitrogen-fixing cyanobacterium, whose vegetative cells can mature in four developmental directions. The particular developmental direction is determined by environmental signals. The vegetative cell cycle is maintained when nutrients are sufficient. Limitation for combined nitrogen induces the terminal differentiation of heterocysts, cells specialized for nitrogen fixation in an oxic environment. A number of unique regulatory events and genes have been identified and integrated into a working model of heterocyst differentiation. Phosphate limitation induces the transient differentiation of akinetes, spore-like cells resistant to cold and desiccation. A variety of environmental changes, both positive and negative for growth, induce the transient differentiation of hormogonia, motile filaments that function in dispersal. Initiation of the differentiation of heterocysts, akinetes and hormogonia are hypothesized to depart from the vegetative cell cycle, following separate and distinct events. N. punctiforme also forms nitrogen-fixing symbiotic associations; its plant partners influence the differentiation and behavior of hormogonia and heterocysts. N. punctiforme is genetically tractable and its genome sequence is nearly complete. Thus, the regulatory circuits of three cellular differentiation events and symbiotic interactions of N. punctiforme can be experimentally analyzed by functional genomics.