Mutational analysis of genes involved in pilus structure, motility and transformation competency in the unicellular motile cyanobacterium Synechocystis sp. PCC 6803

Biochemical and functional characterization of BLUF-type flavin-binding proteins of two species of cyanobacteria

BLUF (a sensor of Blue-Light Using FAD) is a novel putative photoreceptor domain that is found in many bacteria and some eukaryotic algae. As found on genome analysis, certain cyanobacteria have BLUF proteins with a short C-terminal extension. As typical examples, Tll0078 from thermophilic Thermosynechococcus elongatus BP-1 and Slr1694 from mesophilic Synechocystis sp. PCC 6803 were comparatively studied. FAD of both proteins was hardly reduced by exogenous reductants or mediators except methylviologen but showed a typical spectral shift to a longer wavelength upon excitation with blue light. In particular, freshly prepared Tll0078 protein showed slow but reversible aggregation, indicative of light-induced conformational changes in the protein structure. Tll0078 is far more stable as to heat treatment than Slr1694, as judged from flavin fluorescence. The slr1694-disruptant showed phototactic motility away from the light source (negative phototaxis), while the wild type Synechocystis showed positive phototaxis toward the source. Yeast two-hybrid screening with slr1694 showed self-interaction of Slr1694 (PixD) with itself and interaction with a novel PatA-like response regulator, Slr1693 (PixE). These results were discussed in relation to the signaling mechanism of the "short" BLUF proteins in the regulation of cyanobacterial phototaxis.

Novel adaptive responses revealed by transcription profiling of a Synechocystis sp. PCC 6803 delta-isiA mutant in the presence and absence of hydrogen peroxide

The isiAB genes have proven to be highly stress-responsive under a variety of environmental conditions, including iron deficiency, high salt and oxidative stress. In order to understand the function of IsiA and its importance in oxidative stress, we constructed a knock out mutant of the isiA gene and compared differential gene expression of the DeltaisiA strain in the presence and absence of H2O2. We used the full genome microarray for the cyanobacterium Synechocystis sp. PCC 6803 as previously described [Postier BL, Wang HL, Singh A, Impson L, Andrews, HL, Klahn J, Li H, Risinger G, Pesta D, Deyholos M, Galbraith DW, Sherman LA and Burnap RL (2003) BMC Genenomics 4: 23-34]. We determined that one of the main differences in DeltaisiA compared to wild-type (in the absence of peroxide) was the induction of a gene cluster (sll1693-sll1696) that encoded genes resembling pilins or general secretory proteins (Gsp). These proteins are targeted to the cytoplasmic membrane and we suggest that they may be involved in the assembly of membrane complexes, including pigment-protein complexes. The DeltaisiA strain was more resistant to H2O2 compared to the wild-type. In the presence of 1.5 mM H2O2 for 30 min, a cluster of genes that includes a peroxiredoxin was induced 7- to 8-fold and we suggest that this peroxide scavenging enzyme is responsible for the increased peroxide resistance of the DeltaisiA strain.

Cyanobacterial Phytochrome-like PixJ1 Holoprotein Shows Novel Reversible Photoconversion Between Blue- and Green-absorbing Forms

The gene, pixJ1 (formerly pisJ1), is predicted to encode a phytochrome-like photoreceptor that is essential for positive phototaxis in the unicellular cyanobacterium Synechocystis sp. PCC 6803 [Yoshihara et al. (2000) Plant Cell Physiol. 41: 1299]. The PixJ1 protein was overexpressed as a fusion with a poly-histidine tag (His-PixJ1) and isolated from Synechocystis cells. A zinc-fluorescence assay suggested that a linear tetrapyrrole was covalently attached to the His-PixJ1 protein as a chromophore. His-PixJ1 showed novel photoreversible conversion between a blue light-absorbing form (Pb, lambdaAmax=425-435 nm) and a green light-absorbing form (Pg, lambdaAmax=535 nm). Dark incubation led Pg to revert to Pb, indicative of stability of the Pb form in darkness. Red or far-red light irradiation, which is effective for photochemical conversion of the known phytochromes, produced no change in the spectra of Pb and Pg forms. Site-directed mutagenesis revealed that a Cys-His motif in the second GAF domain of PixJ1 is responsible for binding of the chromophore. Possible chromophore species are discussed with regard to the novel photoconversion spectrum.

Light matters: phototaxis and signal transduction in unicellular cyanobacteria

Many photosynthetic microorganisms have evolved the ability to sense light quality and/or quantity and can steer themselves into optimal conditions within the environment. Phototaxis and gliding motility in unicellular cyanobacteria require type IV pili, which are multifunctional cell surface appendages. Screens for cells exhibiting aberrant motility uncovered several non-motile mutants as well as some that had lost positive phototaxis (consequently, they were negatively phototactic). Several negatively phototactic mutants mapped to the tax1 locus, which contains five chemotaxis-like genes. This locus includes a gene that encodes a putative photoreceptor (TaxD1) for positive phototaxis. A second chemotaxis-like cluster (tax3 locus) appears to be involved in pilus biogenesis. The biosynthesis and regulation of type IV pilus-based motility as well as the communication between the pilus motor and photosensory molecules appear to be complex and tightly regulated. Furthermore, the discovery that cyclic AMP and novel gene products are necessary for phototaxis/motility suggests that there might be additional levels of communication and signal processing.

The LuxR homolog ExpR, in combination with the Sin quorum sensing system, plays a central role in Sinorhizobium meliloti gene expression

Quorum sensing, a population density-dependent mechanism for bacterial communication and gene regulation, plays a crucial role in the symbiosis between alfalfa and its symbiont Sinorhizobium meliloti. The Sin system, one of three quorum sensing systems present in S. meliloti, controls the production of the symbiotically active exopolysaccharide EPS II. Based on DNA microarray data, the Sin system also seems to regulate a multitude of S. meliloti genes, including genes that participate in low-molecular-weight succinoglycan production, motility, and chemotaxis, as well as other cellular processes. Most of the regulation by the Sin system is dependent on the presence of the ExpR regulator, a LuxR homolog. Gene expression profiling data indicate that ExpR participates in additional cellular processes that include nitrogen fixation, metabolism, and metal transport. Based on our microarray analysis we propose a model for the regulation of gene expression by the Sin/ExpR quorum sensing system and another possible quorum sensing system(s) in S. meliloti.

Isolation of Outer Membrane of Synechocystis sp. PCC 6803 and Its Proteomic Characterization

In this report, we describe a newly developed method for isolating outer membranes from Synechocystis sp. PCC 6803 cells. The purity of the outer membrane fraction was verified by immunoblot analysis using antibodies against membrane-specific marker proteins. We investigated the protein composition of the outer membrane using two-dimensional gel electrophoresis and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry followed by database identification. Forty-nine proteins were identified corresponding to 29 different gene products. All of the identified proteins have a putative N-terminal signal peptide. About 40% of the proteins identified represent hypothetical proteins with unknown function. Among the proteins identified are a Toc75 homologue, a protein that was initially found in the outer envelope of chloroplasts in pea, as well as TolC, putative porins, and a pilus protein. Other proteins identified include ABC transporters and GumB, which has a suggested function in carbohydrate export. A number of proteases such as HtrA were also found in the outer membrane of Synechocystis sp. PCC 6803.

Improved Genetic Transformation of the Thermophilic Cyanobacterium, Thermosynechococcus elongatus BP-1

We improved genetic transformation of the thermophilic cyanobacterium, Thermosynechococcus elongatus BP-1, by combining electroporation with a top agar method. Transformation was also improved when a disruptant of a putative type I restriction endonuclease (tll2230) was used as recipient cells. In particular, some constructs, with which wild type has never been transformed, were successfully integrated into the tll2230-disruptant. Single-crossover recombination was detected more frequently than the double-crossover recombination. In accordance with the presence of all the homologs of pil genes in Synechocystis sp. PCC 6803, we found that T. elongatus is naturally transformable with exogenous DNA.

The role of Slr1443 in pilus biogenesis in Synechocystis sp. PCC 6803: involvement in post-translational modification of pilins

We isolated a transposon-induced nonmotile mutant of the cyanobacterium Synechocystis sp. PCC 6803. The mutant was revealed to have a Tn5 insertion in the slr1443 gene that showed sequence similarity to a eukaryotic-type protein kinase. Thick pili were not observed on the mutant cell surface under the electron microscope. The slr1443 gene was not involved in transcription or translation of the pilA1 gene encoding pilin, the major component of thick pili. In the mutant, lower molecular mass pilin peptides were detected than in the wild-type. The pilin variant was not truncated at the N- or C-terminus of mature PilA1. The reduced molecular mass may have resulted from insufficient post-translational modification. The amounts of pilin variants were remarkably reduced in the periplasmic and surface fractions. The pilin variants were released into liquid media without being assembled into pili. Our finding suggests that Slr1443 plays an important role in pilus biogenesis at the level of the post-translational modification of pilin.

Pulling Together with Type IV Pili

Type IV pili are an efficient and versatile device for bacterial surface motility. They are widespread among the beta-, gamma-, and delta-proteobacteria and the cyanobacteria. Within that diversity, there is a core of conserved proteins that includes the pilin (PilA), the motors PilB and PilT, and various components of pilus biogenesis and assembly, PilC, PilD, PilM, PilN, PilO, PilP, and PilQ. Progress has been made in understanding the motor and the secretory functions. PilT is a motor protein that catalyzes pilus retraction; PilB may play a similar role in pilus extension. Type IV pili are multifunctional complexes that can act as bacterial virulence factors because pilus-based motility is used to spread pathogens over the surface of a tissue, or to build multicellular structures such as biofilms and fruiting bodies.

Pilin-like proteins in the extremely thermophilic bacterium Thermus thermophilus HB27: implication in competence for natural transformation and links to type IV pilus biogenesis

The extreme thermophile Thermus thermophilus HB27 exhibits high frequencies of natural transformation. Although we recently reported identification of the first competence genes in Thermus, the molecular basis of DNA uptake is unknown. A pilus-like structure is assumed to be involved. Twelve genes encoding prepilin-like proteins were identified in three loci in the genome of T. thermophilus. Mutational analyses, described in this paper, revealed that one locus, which contains four genes that encode prepilin-like proteins (pilA1 to pilA4), is essential for natural transformation. Additionally, comZ, a new competence gene with no similarity to known genes, was identified. Analysis of the piliation phenotype revealed wild-type piliation of a pilA1-pilA3Deltakat mutant and a comZ mutant, whereas a pilA4 mutant was found to be completely devoid of pilus structures. These findings, together with the significant similarity of PilA4 to prepilins, led to the conclusion that the T. thermophilus pilus structures are type IV pili. Furthermore, the loss of the transformation and piliation phenotype in the pilA4 mutant suggests that type IV pili are implicated in natural transformation of T. thermophilus HB27.

Cyanobacterial leader peptides for protein secretion

The leader peptide of the major secreted protein PilA1 of the cyanobacterium Synechocystis sp. strain PCC 6803 and several artificial leader peptides have been used to study secretion of the reporter protein lichenase to the culture medium. The strains of Synechocystis carrying lichenase with the leader sequences of PilA and with the leader sequence of Slr2016 efficiently secreted the reporter protein. The artificial leader sequence that was characterized by the overall positive charge (as PilA1 and Slr2016 leaders) also allowed secretion. The artificial leader with negative charge, however, did not allow secretion of the reporter protein. Moreover, no secreted proteins have been isolated from this strain using conventional techniques for preparation of secreted proteins. These data suggest that the general secretion pathway in cyanobacteria, at least for pilins, recognizes the overall charge of the leader sequences, and operates in a sequence-non-specific manner.

Type IV pili and twitching motility

Twitching motility is a flagella-independent form of bacterial translocation over moist surfaces. It occurs by the extension, tethering, and then retraction of polar type IV pili, which operate in a manner similar to a grappling hook. Twitching motility is equivalent to social gliding motility in Myxococcus xanthus and is important in host colonization by a wide range of plant and animal pathogens, as well as in the formation of biofilms and fruiting bodies. The biogenesis and function of type IV pili is controlled by a large number of genes, almost 40 of which have been identified in Pseudomonas aeruginosa. A number of genes required for pili assembly are homologous to genes involved in type II protein secretion and competence for DNA uptake, suggesting that these systems share a common architecture. Twitching motility is also controlled by a range of signal transduction systems, including two-component sensor-regulators and a complex chemosensory system.

Bipolar localization of putative photoreceptor protein for phototaxis in thermophilic cyanobacterium Synechococcus elongatus

We identified an open reading frame from a database of the entire genome of Synechococcus elongatus, the product of which was very similar to pixJ1, which was proposed as photoreceptor gene for phototaxis in Synechocystis sp. PCC6803 [Yoshihara et al. (2000) Plant Cell Physiol. 41: 1299]. The mRNA of S. elongatus pixJ (SepixJ) was expressed in vivo as a part of the product of an operon. SePixJ was detected exclusively in the membrane fraction after cell fractionation. Immunogold labeling of SePixJ in ultra-thin sections indicated that it existed only in both ends of the rod-shaped cell; probably bound with the cytoplasmic membrane.

Proteomics of Synechocystis sp. strain PCC 6803: identification of plasma membrane proteins

Cyanobacteria are unique prokaryotes since they in addition to outer and plasma membranes contain the photosynthetic membranes (thylakoids). The plasma membranes of Synechocystis 6803, which can be completely purified by density centrifugation and polymer two-phase partitioning, have been found to be more complex than previously anticipated, i.e. they appear to be essential for assembly of the two photosystems. A proteomic approach for the characterization of cyanobacterial plasma membranes using two-dimensional gel electrophoresis and mass spectrometry analysis revealed a total of 57 different membrane proteins of which 17 are integral membrane spanning proteins. Among the 40 peripheral proteins 20 are located on the periplasmic side of the membrane, while 20 are on the cytoplasmic side. Among the proteins identified are subunits of the two photosystems as well as Vipp1, which has been suggested to be involved in vesicular transport between plasma and thylakoid membranes and is thus relevant to the possibility that plasma membranes are the initial site for photosystem biogenesis. Four subunits of the Pilus complex responsible for cell motility were also identified as well as several subunits of the TolC and TonB transport systems. Several periplasmic and ATP-binding proteins of ATP-binding cassette transporters were also identified as were two subunits of the F(0) membrane part of the ATP synthase.

Competence for natural transformation in Neisseria gonorrhoeae: components of DNA binding and uptake linked to type IV pilus expression

The mechanisms by which DNA is taken up into the bacterial cell during natural genetic transformation are poorly understood. Although related components essential to the uptake of DNA during transformation have been defined in Gram-negative species, it remains unclear whether DNA binding and uptake are dissociable events. Therefore, DNA uptake has been the earliest definable step in any Gram-negative transformation pathway. In the human pathogen Neisseria gonorrhoeae, sequence-specific DNA uptake requires an intact type IV pili (Tfp) biogenesis machinery along with three molecules that are dispensable for Tfp expression: ComP (a pilin subunit-like molecule), PilT (a cytoplasmic protein involved in pilus retraction) and ComE (a periplasmic protein with intrinsic DNA-binding activity). By conditionally altering the levels of ComP and PilT expression, we show here that DNA binding and uptake are resolvable events. Consequently, we are able to demonstrate that PilT is largely dispensable for functional DNA binding and, therefore, contributes specifically to uptake. Furthermore, sequence specificity in this system is imposed at the level of DNA binding, a process that is influenced by both ComP and PilE. However, sequence-specific DNA binding is not attributable to an intrinsic property of the Tfp subunit protein. Finally, we demonstrate the existence of a robust, non-specific DNA-binding activity associated with the expression of both Tfp and PilT, which is unrelated to transformation but obscures the observation of specific binding events.

pilG Gene cluster and split pilL genes involved in pilus biogenesis, motility and genetic transformation in the cyanobacterium Synechocystis sp. PCC 6803

The unicellular motile cyanobacterium Synechocystis sp. PCC 6803 exhibits phototactic motility that depends on the type IV-like thick pilus structure. By gene disruption analysis, we showed that a gene cluster of slr1041, slr1042, slr1043 and slr1044, whose predicted products are homologous to PatA, CheY, CheW and MCP, respectively, was more or less required for pilus assembly, motility and natural transformation competency with extraneous DNA. By sequence homology, the missing cheA-like gene in this cluster was identified as novel split genes, slr0073 and slr0322, at separate loci on the genome. This was confirmed by non-motile phenotype of their disruptants. Unique hyperpiliation was observed in the slr1042 and slr0073 disruptants, suggestive of their specific interaction with pilT1. The genes, thus identified as pil genes in this study, were designated pilG (slr1041), pilH (slr1042), pilI (slr1043), pilJ (slr1044), pilL-N (slr0073) and pilL-C (slr0322).

A cAMP receptor protein, SYCRP1, is responsible for the cell motility of Synechocystis sp. PCC 6803

Disruption of the sycrp1 gene encoding a cyanobacterial cAMP receptor protein makes cells of Synechocystis sp. PCC 6803 non-motile. Electron microscopy showed that the sycrp1-deficient strain had a reduced number of thick pili on the cell surface compared with the wild-type strain. It is suggested that cAMP-SYCRP1 complex controls the biogenesis of pili.

Molecular analyses of the natural transformation machinery and identification of pilus structures in the extremely thermophilic bacterium Thermus thermophilus strain HB27

Thermus thermophilus HB27, an extremely thermophilic bacterium, exhibits high competence for natural transformation. To identify genes of the natural transformation machinery of T. thermophilus HB27, we performed homology searches in the partially completed T. thermophilus genomic sequence for conserved competence genes. These analyses resulted in the detection of 28 open reading frames (ORFs) exhibiting significant similarities to known competence proteins of gram-negative and gram-positive bacteria. Disruption of 15 selected potential competence genes led to the identification of 8 noncompetent mutants and one transformation-deficient mutant with a 100-fold reduced transformation frequency. One competence protein is similar to DprA of Haemophilus influenzae, seven are similar to type IV pilus proteins of Pseudomonas aeruginosa or Neisseria gonorrhoeae (PilM, PilN, PilO, PilQ, PilF, PilC, PilD), and another deduced protein (PilW) is similar to a protein of unknown function in Deinococcus radiodurans R1. Analysis of the piliation phenotype of T. thermophilus HB27 revealed the presence of single pilus structures on the surface of the wild-type cells, whereas the noncompetent pil mutants of Thermus, with the exception of the pilF mutant, were devoid of pilus structures. These results suggest that pili and natural transformation in T. thermophilus HB27 are functionally linked.

A eukaryotic-type protein kinase, SpkA, is required for normal motility of the unicellular Cyanobacterium synechocystis sp. strain PCC 6803

The genome of the unicellular cyanobacterium Synechocystis sp. strain PCC 6803 comprises many open reading frames (ORFs) which putatively encode eukaryotic-type protein kinase and protein phosphatase. Based on gene disruption analysis, a region of the hypothetical ORF sll1575, which retained a part of the protein kinase motif, was found to be required for normal motility in the original isolate of strain PCC 6803. Sequence determination revealed that in this strain sll1575 was part of a gene (designated spkA) which harbored an entire eukaryotic-type Ser/Thr protein kinase motif. Strain ATCC 27184 and a glucose-tolerant strain derived from the same isolate as the PCC strain had a frameshift mutation dividing spkA into ORFs sll1574 and sll1575. The structural integrity of spkA agreed well with the motility phenotype, determined by colony morphology on agar plates. The spkA gene was expressed in Escherichia coli as a His-tagged protein, which was purified by Ni2+ affinity chromatography. With [gamma-32P]ATP, SpkA was autophosphorylated and transferred the phosphate group to casein, myelin basic protein, and histone. SpkA also phosphorylated several proteins in the membrane fraction of Synechocystis cells. These results suggest that SpkA is a eukaryotic-type Ser/Thr protein kinase and regulates cellular motility via phosphorylation of the membrane proteins in Synechocystis.

Novel putative photoreceptor and regulatory genes Required for the positive phototactic movement of the unicellular motile cyanobacterium Synechocystis sp. PCC 6803

Synechocystis: sp. PCC 6803 is a unicellular motile cyanobacterium, which shows positive or negative phototaxis on agar plates under lateral illumination. By gene disruption in a substrain showing of positive phototaxis, it was demonstrated that mutants defective in sll0038, sll0039, sll0041, sll0042 or sll0043 lost positive phototaxis but showed negative phototaxis away from the light source. Mutants of sll0040, which is located within the cluster of these genes, retained the capacity of positive phototaxis but to a lesser extent than the parent cells. These genes are homologous to che genes, which are involved in flagellar switching for bacterial chemotaxis. Interestingly, sll0041 (designated pisJ1) is predicted to have a chromophore-binding motif of phytochrome-like proteins and a signaling motif of chemoreceptors for bacterial chemotaxis. It is strongly suggested that the positive phototactic response was mediated by a phytochrome-like photoreceptor and CheA/CheY-type signal transduction system.