Vectors for determining the differential expression of genes in heterocysts and vegetative cells of Anabaena sp. strain PCC 7120

Enhancing Light-Driven 1,3-Propanediol Production by Using Natural Compartmentalization of Differentiated Cells

Synthetic biology emerges as a powerful approach for unlocking the potential of cyanobacteria to produce various chemicals. However, the highly oxidative intracellular environment of cyanobacteria is incompatible to numerous introduced enzymes from anaerobes. In this study, we explore a strategy based on natural compartmentalization of cyanobacterial heterocysts to overcome the incompatibility. Hence, the oxygen-sensitive 1,3-propanediol (1,3-PDO) biosynthetic pathway was selected as a model and insulated in heterocysts to evaluate the proposed strategy. Thus, the genes from different sources for 1,3-PDO production were tandemly arrayed with promoter, resulting the assembled 1,3-PDO synthetic cassettes. Then the synthetic cassettes were integrated into the chromosome of Anabaena sp. strain PCC7120 by homologous recombination, respectively. The engineered strain P11 containing the genes from facultative anaerobe Klebsiella pneumoniae (cassette KP) accumulated 46.0 mg L^-1 of 1,3-PDO when heterocysts were present, which is approximately 1.7-fold higher than that of no heterocysts. As for the strains (P12, P13, and P14) containing the genes from strictly anaerobic bacterium Clostridium butyricum (cassette CB), the product 1,3-PDO could only be detected when heterocysts were present. These results indicate that insulation of the oxygen-sensitive 1,3-PDO pathway with heterocysts is an effective way to protect these enzymes in cyanobacteria. The strategy may have the potential of serving as a universal strategy to overcome the incompatibility of oxygen-sensitivity in synthetic biology.

Site-directed mutagenesis of the Anabaena sp. strain PCC 7120 nitrogenase active site to increase photobiological hydrogen production

Cyanobacteria use sunlight and water to produce hydrogen gas (H₂), which is potentially useful as a clean and renewable biofuel. Photobiological H₂ arises primarily as an inevitable by-product of N₂ fixation by nitrogenase, an oxygen-labile enzyme typically containing an iron-molybdenum cofactor (FeMo-co) active site. In Anabaena sp. strain 7120, the enzyme is localized to the microaerobic environment of heterocysts, a highly differentiated subset of the filamentous cells. In an effort to increase H₂ production by this strain, six nitrogenase amino acid residues predicted to reside within 5 Å of the FeMo-co were mutated in an attempt to direct electron flow selectively toward proton reduction in the presence of N₂. Most of the 49 variants examined were deficient in N₂-fixing growth and exhibited decreases in their in vivo rates of acetylene reduction. Of greater interest, several variants examined under an N₂ atmosphere significantly increased their in vivo rates of H₂ production, approximating rates equivalent to those under an Ar atmosphere, and accumulated high levels of H₂ compared to the reference strains. These results demonstrate the feasibility of engineering cyanobacterial strains for enhanced photobiological production of H₂ in an aerobic, nitrogen-containing environment.

Cellular differentiation and the NtcA transcription factor in filamentous cyanobacteria

Some filamentous cyanobacteria can undergo a variety of cellular differentiation processes that permit their better adaptation to certain environmental conditions. These processes include the differentiation of hormogonia, short filaments aimed at the dispersal of the organism in the environment, of akinetes, cells resistant to various stress conditions, and of heterocysts, cells specialized in the fixation of atmospheric nitrogen in oxic environments. NtcA is a transcriptional regulator that operates global nitrogen control in cyanobacteria by activating (and in some cases repressing) many genes involved in nitrogen assimilation. NtcA is required for the triggering of heterocyst differentiation and for subsequent steps of its development and function. This requirement is based on the role of NtcA as an activator of the expression of hetR and other multiple genes at specific steps of the differentiation process. The products of these genes effect development as well as the distinct metabolism of the mature heterocyst. The different features found in the NtcA-dependent promoters, together with the cellular level of active NtcA protein, should have a role in the determination of the hierarchy of gene activation during the process of heterocyst differentiation.

Identification and inactivation of three group 2 sigma factor genes in Anabaena sp. strain PCC 7120

Three new Anabaena sp. strain PCC 7120 genes encoding group 2 alternative sigma factors have been cloned and characterized. Insertional inactivation of sigD, sigE, and sigF genes did not affect growth on nitrate under standard laboratory conditions but did transiently impair the abilities of sigD and sigE mutant strains to establish diazotrophic growth. A sigD sigE double mutant, though proficient in growth on nitrate and still able to differentiate into distinct proheterocysts, was unable to grow diazotrophically due to extensive fragmentation of filaments upon nitrogen deprivation. This double mutant could be complemented by wild-type copies of sigD or sigE, indicating some degree of functional redundancy that can partially mask phenotypes of single gene mutants. However, the sigE gene was required for lysogenic development of the temperate cyanophage A-4L. Several other combinations of double mutations, especially sigE sigF, caused a transient defect in establishing diazotrophic growth, manifested as a strong and prolonged bleaching response to nitrogen deprivation. We found no evidence for developmental regulation of the sigma factor genes. luxAB reporter fusions with sigD, sigE, and sigF all showed slightly reduced expression after induction of heterocyst development by nitrogen stepdown. Phylogenetic analysis of cyanobacterial group 2 sigma factor sequences revealed that they fall into several subgroups. Three morphologically and physiologically distant strains, Anabaena sp. strain PCC 7120, Synechococcus sp. strain PCC 7002, and Synechocystis sp. strain PCC 6803 each contain representatives of four subgroups. Unlike unicellular strains, Anabaena sp. strain PCC 7120 has three additional group 2 sigma factors that cluster in subgroup 2.5b, which is perhaps specific for filamentous or heterocystous cyanobacteria.

The hetC gene is a direct target of the NtcA transcriptional regulator in cyanobacterial heterocyst development

The heterocyst is the site of nitrogen fixation in aerobically grown cultures of some filamentous cyanobacteria. Heterocyst development in Anabaena sp. strain PCC 7120 is dependent on the global nitrogen regulator NtcA and requires, among others, the products of the hetR and hetC genes. Expression of hetC, tested by RNA- DNA hybridization, was impaired in an ntcA mutant. A nitrogen-regulated, NtcA-dependent putative transcription start point was localized at nucleotide -571 with respect to the hetC translational start. Sequences upstream from this transcription start point exhibit the structure of the canonical cyanobacterial promoter activated by NtcA, and purified NtcA protein specifically bound to a DNA fragment containing this promoter. Activation of expression of hetC during heterocyst development appears thus to be directly operated by NtcA. NtcA-mediated activation of hetR expression was not impaired in a hetC mutant, indicating that HetC is not an NtcA-dependent element required for hetR induction.

Heterocyst formation

Heterocysts are microaerobic, N2-fixing cells that form in a patterned array within O2-producing filamentous cyanobacteria. Structural features of heterocysts can be predicted from consideration of their physiology. This review focuses on the spacing mechanism that determines which cells will differentiate, and on the regulation of the progression of the differentiation process. Applicable genetic tools, developed primarily using Anabaena PCC 7120, but employed also with Nostoc spp., are reviewed. These tools include localization of transcription using fusions to lux, lac, and gfp, and mutagenesis with oriV-containing derivatives of transposon Tn5. Mature and developing heterocysts inhibit nearby vegetative cells from differentiating; genes patA, devA, hetC, and the hetMNI locus may hold keys to understanding intercellular interactions that influence heterocyst formation. Regulatory and other genes that are transcriptionally activated at different times after nitrogen stepdown have been identified, and should permit analysis of mechanisms that underlie the progression of heterocyst differentiation.

Transcription of the Anabaena sp. strain PCC 7120 ntcA gene: multiple transcripts and NtcA binding

The Anabaena sp. strain PCC 7120 ntcA gene showed multiple transcripts with different 5' ends. The relative abundance of transcripts varied in response to nitrogen availability. The ntcA product, NtcA, showed binding to the promoter region of its own gene. The binding site mapped to a region between the transcription start site used under nitrogen-replete conditions and the start sites used under nitrogen-limiting conditions, suggesting that NtcA regulates its own expression.