A DNA fragment from the cyanobacterium Synechocystis sp. PCC 6803 mediates gene expression inducible by osmotic stress in E. coli

Design and analysis of LacI-repressed promoters and DNA-looping in a cyanobacterium

BACKGROUND

Cyanobacteria are solar-powered prokaryotes useful for sustainable production of valuable molecules, but orthogonal and regulated promoters are lacking. The Lac repressor (LacI) from Escherichia coli is a well-studied transcription factor that is orthogonal to cyanobacteria and represses transcription by binding a primary lac operator (lacO), blocking RNA-polymerase. Repression can be enhanced through DNA-looping, when a LacI-tetramer binds two spatially separated lacO and loops the DNA. Ptrc is a commonly used LacI-repressed promoter that is inefficiently repressed in the cyanobacterium Synechocystis PCC 6803. Ptrc2O, a version of Ptrc with two lacO, is more efficiently repressed, indicating DNA-looping. To investigate the inefficient repression of Ptrc and cyanobacterial DNA-looping, we designed a Ptrc-derived promoter library consisting of single lacO promoters, including a version of Ptrc with a stronger lacO (Ptrc1O-proximal), and dual lacO promoters with varying inter-lacO distances (the Ptrc2O-library).

RESULTS

We first characterized artificial constitutive promoters and used one for engineering a LacI-expressing strain of Synechocystis. Using this strain, we observed that Ptrc1O-proximal is similar to Ptrc in being inefficiently repressed. Further, the Ptrc2O-library displays a periodic repression pattern that remains for both non- and induced conditions and decreases with longer inter-lacO distances, in both E. coli and Synechocystis. Repression of Ptrc2O-library promoters with operators out of phase is less efficient in Synechocystis than in E. coli, whereas repression of promoters with lacO in phase is efficient even under induced conditions in Synechocystis. Two well-repressed Ptrc2O promoters were highly active when tested in absence of LacI in Synechocystis.

CONCLUSIONS

The artificial constitutive promoters herein characterized can be utilized for expression in cyanobacteria, as demonstrated for LacI. The inefficient repression of Ptrc and Ptrc1O-proximal in Synechocystis, as compared to E. coli, may be due to insufficient LacI expression, or differences in RNAP subunits. DNA-looping works as a transcriptional regulation mechanism similarly as in E. coli. DNA-looping contributes strongly to Ptrc2O-library repression in Synechocystis, even though they contain the weakly-repressed primary lacO of Ptrc1O-proximal and relatively low levels of LacI/cell. Hence, Synechocystis RNAP may be more sensitive to DNA-looping than E. coli RNAP, and/or the chromatin torsion resistance could be lower. Two strong and highly repressed Ptrc2O promoters could be used without induction, or together with an unstable LacI.

Construction and use of GFP reporter vectors for analysis of cell-type-specific gene expression in Nostoc punctiforme

Two transcriptional reporter shuttle vectors were constructed for the filamentous cyanobacterium Nostoc punctiforme using the green fluorescence protein (GFP) reporter. Both the ampicillin- and kanamycin-resistant versions of the plasmid allow promoters to be directionally cloned into a multiple cloning site preceding a promoterless gfp gene using an Escherichia coli host. The ability of the self-replicating shuttle plasmids to report cell-type-specific gene expression in N. punctiforme was tested by cloning promoters expressed in normal vegetative cells, nitrogen-fixing heterocysts and spore-like akinetes. A P(psaC) reporter gene fusion was expressed in vegetative cells and not in heterocysts, whereas GFP driven from P(hetR) was found highly expressed in heterocysts. GFP expression driven by the promoter for the N. punctiforme homologue of the akinete-specific gene avaK was expressed in developing akinetes. Decreased expression of GFP from the P(psaC) reporter in hormogonia was also observed. The results demonstrate the utility of these GFP vectors to study cell-type-specific gene expression in differentiating filamentous cyanobacteria.

Chimeras of P-type ATPases and their transcriptional regulators: contributions of a cytosolic amino-terminal domain to metal specificity

Zn(2+)-responsive repressor ZiaR and Co(2+)-responsive activator CoaR modulate production of P(1)-type Zn(2+)- (ZiaA) and Co(2+)- (CoaT) ATPases respectively. What dictates metal selectivity? We show that Delta ziaDeltacoa double mutants had similar Zn(2+) resistance to Deltazia single mutants and similar Co(2+) resistance to Deltacoa single mutants. Controlling either ziaA or coaT with opposing regulators restored no resistance to metals sensed by the regulators, but coincident replacement of the deduced cytosolic amino-terminal domain CoaT(N) with ZiaA(N) (in ziaR-(p) ziaA-ziaA(N)coaT) conferred Zn(2+) resistance to DeltaziaDeltacoa, Zn(2+) content was lowered and residual Co(2+) resistance lost. Metal-dependent molar absorptivity under anaerobic conditions revealed that purified ZiaA(N) binds Co(2+) in a pseudotetrahedral two-thiol site, and Co(2+) was displaced by Zn(2+). Thus, the amino-terminal domain of ZiaA inverts the metals exported by zinc-regulated CoaT from Co(2+) to Zn(2+), and this correlates simplistically with metal-binding preferences; K(ZiaAN) Zn(2+) tighter than Co(2+). However, Zn(2+) did not bleach Cu(+)-ZiaA(N), and only Cu(+) co-migrated with ZiaA(N) after competitive binding versus Zn(2+). Bacterial two-hybrid assays that detected interaction between the Cu(+)-metallochaperone Atx1 and the amino-terminal domain of Cu(+)-transporter PacS(N) detected no interaction with the analogous, deduced, ferredoxin-fold subdomain of ZiaA(N). Provided that there is no freely exchangeable cytosolic Cu(+), restricted contact with the Cu(+)-metallochaperone can impose a barrier impairing the formation of otherwise favoured Cu(+)-ZiaA(N) complexes.

Construction of promoter probe vectors for Synechocystis sp. PCC 6803 using the light-emitting reporter systems Gfp and LuxAB

Two promoter probe vectors were constructed for the cyanobacterium Synechocystis sp. strain PCC 6803 using reporter genes, which can be easily detected and quantified in vivo by the ability of their encoded proteins to emit light. The vectors allow the transcriptional fusion of promoter sequences with the gfp and luxAB genes, respectively, and their stable integration into a neutral site of the Synechocystis chromosome. Functionality of these vectors was demonstrated by cloning the promoter of the isiAB operon into both promoter probe vectors and analyzing the stress-dependent emission of light by the obtained reporter strains. As was found before for the isiAB operon, the P(isiAB) reporter gene fusions were induced by iron starvation and high salt stress. Induction rates of mRNA of the wild type operon and the reporter gene fusions were found to be essentially the same, indicating that a promoter fragment containing all necessary regulatory elements has been cloned. However, using the gfp gene a slow increase of protein and fluorescence was found, while the luxAB reporter gene constructs led to a rapid increase in luminescence. The same was found after retransfer of cells back into control media, in which the Gfp protein disappeared slowly, while the LuxAB-based luminescence decreased rapidly. These experiments show that both reporter genes can be used in Synechocystis: the luxAB system seems to be favourable regarding reaction time, while the gfp system has the advantage of being independent from any substrate.

Cobalt-dependent transcriptional switching by a dual-effector MerR-like protein regulates a cobalt-exporting variant CPx-type ATPase

CoaR associates with and confers cobalt-dependent activation of the coaT operator-promoter. A CoaR mutant (Ser-Asn-Ser) in a carboxyl-terminal Cys-His-Cys motif bound the coaT operator-promoter but did not activate expression in response to cobalt, implicating thiolate and/or imidazole ligands at these residues in an allosteric cobalt binding site. Deletion of 1 or 2 nucleotides from between near consensus, but with aberrant (20 base pairs) spacing, -10 and -35 elements enhanced expression from the coaT operator-promoter but abolished activation by cobalt-CoaR. It is inferred that cobalt effects a transition in CoaR that underwinds the coaT operator-promoter to realign promoter elements. In the absence of cobalt, CoaR represses expression (approximately 50%). CoaR is a fusion of ancestral MerR (mercury-responsive transcriptional activator)- and precorrin isomerase (enzyme of vitamin B(12) biosynthesis)-related sequences. Expression from the coaT operator-promoter was enhanced in a partial mutant of cbiE (encoding an enzyme preceding precorrin isomerase in B(12) biosynthesis), revealing that this pathway "inhibits" coaT expression. Disruption of coaT reduced cobalt tolerance and increased cytoplasmic (57)Co accumulation. coaT-mediated restoration of cobalt tolerance has been used as a selectable marker.