No abstract

No abstract

No abstract

Acaryochloris marina is a unique cyanobacterium that is able to produce chlorophyll d as its primary photosynthetic pigment and thus efficiently use far-red light for photosynthesis. Acaryochloris species have been isolated from marine environments in association with other oxygenic phototrophs, which may have driven the niche-filling introduction of chlorophyll d. To investigate these unique adaptations, we have sequenced the complete genome of A. marina. The DNA content of A. marina is composed of 8.3 million base pairs, which is among the largest bacterial genomes sequenced thus far. This large array of genomic data is distributed into nine single-copy plasmids that code for >25% of the putative ORFs. Heavy duplication of genes related to DNA repair and recombination (primarily recA) and transposable elements could account for genetic mobility and genome expansion. We discuss points of interest for the biosynthesis of the unusual pigments chlorophyll d and ␣-carotene and genes responsible for previously studied phycobilin aggregates. Our analysis also reveals that A. marina carries a unique complement of genes for these phycobiliproteins in relation to those coding for antenna proteins related to those in Prochlorococcus species. The global replacement of major photosynthetic pigments appears to have incurred only minimal specializations in reaction center proteins to accommodate these alternate pigments. These features clearly show that the genus Acaryochloris is a fitting candidate for understanding genome expansion, gene acquisition, ecological adaptation, and photosystem modification in the cyanobacteria.comparative microbial genomics ͉ photosynthesis ͉ oxygenic phototrophs ͉ evolution

Despite the fact that heliobacteria are the only phototrophic representatives of the bacterial phylum Firmicutes, genomic analyses of these organisms have yet to be reported. Here we describe the complete sequence and analysis of the genome of Heliobacterium modesticaldum, a thermophilic species belonging to this unique group of phototrophs. The genome is a single 3.1-Mb circular chromosome containing 3,138 open reading frames. As suspected from physiological studies of heliobacteria that have failed to show photoautotrophic growth, genes encoding enzymes for known autotrophic pathways in other phototrophic organisms, including ribulose bisphosphate carboxylase (Calvin cycle), citrate lyase (reverse citric acid cycle), and malyl coenzyme A lyase (3-hydroxypropionate pathway), are not present in the H. modesticaldum genome. Thus, heliobacteria appear to be the only known anaerobic anoxygenic phototrophs that are not capable of autotrophy. Although for some cellular activities, such as nitrogen fixation, there is a full complement of genes in H. modesticaldum, other processes, including carbon metabolism and endosporulation, are more genetically streamlined than they are in most other low-G؉C gram-positive bacteria. Moreover, several genes encoding photosynthetic functions in phototrophic purple bacteria are not present in the heliobacteria. In contrast to the nutritional flexibility of many anoxygenic phototrophs, the complete genome sequence of H. modesticaldum reveals an organism with a notable degree of metabolic specialization and genomic reduction.

Background:In cyanobacteria, light harvesting and photosynthesis occur in the thylakoid membranes. Results:The distances between thylakoid membranes are correlated with the size of the phycobilisome antenna and change reversibly and rapidly upon illumination. Conclusion: Thylakoid membranes have a structural plasticity tied to the regulation of photosynthesis. Significance: Characterizing the structural changes in photosynthetic membranes is crucial for understanding light harvesting and photosynthetic productivity.

The unicellular diazotrophic cyanobacterium Cyanothece sp. ATCC 51142 (Cyanothece 51142) is able to grow aerobically under nitrogen-fixing conditions with alternating light-dark cycles or continuous illumination. This study investigated the effects of carbon and nitrogen sources on Cyanothece 51142 metabolism via 13 C-assisted metabolite analysis and biochemical measurements. Under continuous light (50 mmol photons m "2 s "1 ) and nitrogen-fixing conditions, we found that glycerol addition promoted aerobic biomass growth (by twofold) and nitrogenasedependent hydrogen production [up to 25 mmol H 2 (mg chlorophyll) "1 h "1 ], but strongly reduced phototrophic CO 2 utilization. Under nitrogen-sufficient conditions, Cyanothece 51142 was able to metabolize glycerol photoheterotrophically, and the activity of light-dependent reactions (e.g. oxygen evolution) was not significantly reduced. In contrast, Synechocystis sp. PCC 6803 showed apparent mixotrophic metabolism under similar growth conditions. Isotopomer analysis also detected that Cyanothece 51142 was able to fix CO 2 via anaplerotic pathways, and to take up glucose and pyruvate for mixotrophic biomass synthesis. INTRODUCTIONRising concerns about global warming due to the greenhouse effect have renewed research focused on the biological capture of CO 2 . Cyanobacteria have versatile metabolic capabilities, which allow them to grow under autotrophic, heterotrophic and mixotrophic conditions (Bottomley & Van Baalen, 1978;Eiler, 2006;Yang et al., 2002). More importantly, some cyanobacteria can capture solar energy to fix nitrogen and generate H 2 , thereby serving as a source of biofertilizer and biofuel, while simultaneously consuming atmospheric CO 2 (Bernat et al., 2009;Dutta et al., 2005;Fay, 1992;Madamwar et al., 2000;Tamagnini et al., 2007;Tuli et al., 1996). Cyanothece sp. ATCC 51142 (Cyanothece 51142), a unicellular diazotrophic cyanobacterium, is able to grow aerobically under nitrogen-fixing conditions and has been recognized as contributing to the marine nitrogen cycle. The recent sequencing of the Cyanothece 51142 genome and its transcriptional analysis have uncovered the diurnally oscillatory metabolism of the bacterium in alternating light-dark cycles (photosynthesis during the day and nitrogen fixation at night) (Stöckel et al., 2008;Toepel et al., 2008;Welsh et al., 2008). In general, cyanobacteria use spatial or temporal separation of oxygen-sensitive nitrogen fixation and oxygen-evolving photosynthesis as a strategy for diazotrophic growth (Benemann & Weare, 1974;Fay, 1992). Interestingly, Cyanothece 51142 demonstrates simultaneous N 2 fixation and O 2 evolution under continuous-light conditions, though it appears to be unicellular (Colon-Lopez et al., 1997;Huang & Chow, 1986). For example, a recent study of the transcriptional and translational regulation of continuously illuminated Cyanothece has revealed a strong synthesis capability for nitrogenase and circadian expression of 10 % of its genes (Toepel et al., 2008). Furthermore, Cyanothece str...

Nicotiana section Alatae contains eight species with variable flower sizes and morphologies. Section members readily hybridize in the glasshouse, but no hybrids have been observed in natural sympatric and parapatric populations. To investigate interspecific crossing relationships with respect to mechanisms preventing hybridization, all members of section Alatae were intercrossed in a complete diallel. We found positive correlation between the pistil length of the pollen donor and interspecific seed set relative to the conspecific cross. Pollen tube growth rate and pollen donor pistil length were positively correlated as well. Furthermore, pollen from short-pistil members of section Alatae could only grow a maximum distance proportional to, but greater than, their own pistil lengths. Our results show that pollen tube growth capacity (i.e., rate and distance), provides a hybridization barrier in long-pistil species 9 short-pistil species crosses. We also found another hybridization barrier not specifically related to pollen tube growth capacity in short-pistil species 9 long-pistil species. Taken together, these barriers can generally be described by a 'pistil-length mismatch' rule; in section Alatae, pollen has the most success fertilizing ovules from species with pistil lengths similar to their own. This rule could contribute to hybridization barriers in Section Alatae because the species display dramatically different pistil lengths.