An atypical KdpD homologue from the cyanobacterium Anabaena sp. strain L-31: cloning, in vivo expression, and interaction with Escherichia coli KdpD-CTD

Cyanobacterial KdpD modulates in vivo and in vitro activities of a membrane-anchored histidine kinase

The prokaryotic KdpATPAse complex, encoded by the kdpABC operon, is an inducible, high-affinity K+ transporter. In E. coli, the operon is transcriptionally regulated by a two-component sensor-kinase response-regulator system, constituted by the KdpD and KdpE proteins. In contrast, cyanobacteria exhibit a truncated kdpD gene that encodes a KdpD homolog that is similar to the N-terminal domain (NTD) of E. coli KdpD, but lacks the transmitter, histidine kinase-containing, C-terminal domain (CTD). Here we show that the cyanobacterium Anabaena sp. strain L-31 constitutively transcribes the short kdpD gene, but synthesizes KdpATPase only during potassium starvation. However, unlike E. coli., expression of the kdpD gene remains unaffected by K+ limitation in Anabaena. To gain insight into the possible role of Anabaena KdpD, the chimeric Anacoli KdpD protein, wherein the NTD of E. coli KdpD was replaced with Anabaena KdpD, was functionally analyzed. Detailed investigation has revealed that the Anacoli KdpD (a) responds to a much lower threshold of external K+ than the E. coli KdpD (b) exhibits much reduced ability to induce kdp in response to ionic osmolytes than E. coli KdpD, and is therefore unable to sustain optimal growth in the presence of these osmolytes and (c) displays higher in vitro phosphatase activity than the wild type E. coli KdpD. Thus, Anabaena KdpD modulates properties of E. coli KdpD-CTD in a manner that is quite distinct from the E. coli KdpD-NTD. Based on these evidences, a model for kdp regulation by the short KdpD is proposed.

Potassium deficiency triggers the development of dormant cells (akinetes) in Aphanizomenon ovalisporum (Nostocales, Cyanoprokaryota)(1)

Akinetes are spore-like nonmotile cells that differentiate from vegetative cells of filamentous cyanobacteria from the order Nostocales. They play a key role in the survival and distribution of these species and contribute to their perennial blooms. Various environmental factors were reported to trigger the differentiation of akinetes including light intensity and quality, temperature, and nutrient deficiency. Here, we report that deprivation of potassium ion (K(+) ) triggers akinete development in the cyanobacterium Aphanizomenon ovalisporum. Akinetes formation is initiated 3 d-7 d after an induction by K(+) depletion, followed by 2-3 weeks of a maturation process. Akinete formation occurs within a restricted matrix of environmental conditions such as temperature, light intensity or photon flux. Phosphate is essential for akinete maturation and P-limitation restricts the number of mature akinetes. DNA replication is essential for akinete maturation and akinete development is limited in the presence of Nalidixic acid. While our results unequivocally demonstrated the effect of K(+) deficiency on akinete formation in laboratory cultures of A. ovalisporum, this trigger did not cause Cylindrospermopsis raciborskii to produce akinetes. Anabaena crassa however, produced akinetes upon potassium deficiency, but the highest akinete concentration was achieved at conditions that supported vegetative growth. It is speculated that an unknown internal signal is associated with the cellular response to K(+) deficiency to induce the differentiation of a certain vegetative cell in a trichome into an akinete. A universal stress protein that functions as mediator in K(+) deficiency signal transduction cascade, may communicate between the lack of K(+) and akinete induction.

The complexity of the 'simple' two-component system KdpD/KdpE in Escherichia coli

The KdpD/KdpE two-component system of Escherichia coli activates the expression of the kdpFABC operon encoding the high-affinity K(+) uptake system KdpFABC in response to K(+) limitation or salt stress. Earlier, it was proposed that the histidine kinase KdpD is a turgor sensor; recent studies suggest that KdpD integrates three chemical stimuli from the cytoplasm. The histidine kinase KdpD contains several structural features and subdomains that are important for stimulus perception, modulation of the kinase to phosphatase ratio, and signaling. The response regulator KdpE receives the phosphoryl group from KdpD and induces kdpFABC transcription. The three-dimensional structure of the receiver domain was resolved, providing insights into the activation mechanism of this transcriptional regulator. Two accessory components, the universal stress protein UspC and the phosphotransferase system component IIA(Ntr), are known to interact with KdpD, allowing the modulation of kdpFABC expression under certain physiological conditions. Here, we will discuss the complexity of a 'simple' two-component system and its interconnectivity with metabolism and the general stress response.

Characterization of a response regulator protein that binds to Anabaena sp. strain L-31 kdp-promoter region

The potassium dependent adenosine triphosphatase (Kdp-ATPase), encoded by the kdp operon, is a potassium uptake system found in several prokaryotes. The cyanobacterium Anabaena sp. strain L-31 shows the presence of two kdp operons (kdp1 and kdp2) of which the kdp2 is predominantly induced in response to potassium limitation or desiccation stress. Two ORFs, encoding a sensor kinase and a response regulator, respectively, were identified upstream of the kdp2 operon in Anabaena sp. strain L-31. The response regulator protein, tagged with 6 additional C-terminal histidine residues, was over-expressed in Escherichia coli and purified by affinity chromatography. Employing the protein-specific antiserum, the response regulator protein was detected in Anabaena L-31 cytosolic fractions. The response regulator protein bound to the kdp2 promoter region with higher affinity than kdp1 promoter region. Addition of acetyl phosphate increased the ability of the protein to bind to kdp2 promoter region by several fold, suggesting a phosphorylation-dependent binding of response regulator to the promoter. These data implicate the response regulator protein in regulation of kdp2 expression in Anabaena sp. strain L-31.

The Kdp-ATPase system and its regulation

K+, the dominant intracellular cation, is required for various physiological processes like turgor homeostasis, pH regulation etc. Bacterial cells have evolved many diverse K+ transporters to maintain the desired concentration of internal K+. In E.coli, the KdpATPase (comprising of the KdpFABC complex), encoded by the kdpFABC operon, is an inducible high-affinity K+ transporter that is synthesised under conditions of severe K+ limitation or osmotic upshift. The E.coli kdp expression is transcriptionally regulated by the KdpD and KdpE proteins, which together constitute a typical bacterial two-component signal transduction system. The Kdp system is widely dispersed among the different classes of bacteria including the cyanobacteria. The ordering of the kdpA, kdpB and kdpC is relatively fixed but the kdpD/E genes show different arrangements in distantly related bacteria. Our studies have shown that the cyanobacterium Anabaena sp. strain L-31 possesses two kdp operons, kdp1 and kdp2, of which, the later is expressed under K+ deficiency and desiccation. Among the regulatory genes,the kdpD ORF of Anabaena L-31 is truncated when compared to the kdpD of other bacteria, while a kdpE -like gene is absent. The extremely radio-resistant bacterium, Deinococcus radiodurans strain R1, also shows the presence of a naturally short kdpD ORF similar to Anabaena in its kdp operon. The review elaborates the expression of bacterial kdp operons in response to various environmental stress conditions, with special emphasis on Anabaena. The possible mechanism(s)of regulation of the unique kdp operons from Anabaena and Deinococcus are also discussed.

Cyanobacterial two-component proteins: structure, diversity, distribution, and evolution

A survey of the already characterized and potential two-component protein sequences that exist in the nine complete and seven partially annotated cyanobacterial genome sequences available (as of May 2005) showed that the cyanobacteria possess a much larger repertoire of such proteins than most other bacteria. By analysis of the domain structure of the 1,171 potential histidine kinases, response regulators, and hybrid kinases, many various arrangements of about thirty different modules could be distinguished. The number of two-component proteins is related in part to genome size but also to the variety of physiological properties and ecophysiologies of the different strains. Groups of orthologues were defined, only a few of which have representatives with known physiological functions. Based on comparisons with the proposed phylogenetic relationships between the strains, the orthology groups show that (i) a few genes, some of them clustered on the genome, have been conserved by all species, suggesting their very ancient origin and an essential role for the corresponding proteins, and (ii) duplications, fusions, gene losses, insertions, and deletions, as well as domain shuffling, occurred during evolution, leading to the extant repertoire. These mechanisms are put in perspective with the different genetic properties that cyanobacteria have to achieve genome plasticity. This review is designed to serve as a basis for orienting further research aimed at defining the most ancient regulatory mechanisms and understanding how evolution worked to select and keep the most appropriate systems for cyanobacteria to develop in the quite different environments that they have successfully colonized.

Differential expression of the two kdp operons in the nitrogen-fixing cyanobacterium Anabaena sp. strain L-31

In several types of bacteria, the Kdp ATPase (comprising of the KdpABC complex) is an inducible, high-affinity potassium transporter that scavenges K+ from the environment. The cyanobacterium Anabaena sp. strain L-31 showed the presence of not one but two distinct kdp operons in its genome. The kdp1 consisted of kdpA1B1G1C1D genes, whereas the kdp2 contained the kdpA2B2G2C2 genes. Among the regulatory genes, the kdpD open reading frame of Anabaena sp. strain L-31 was truncated compared to the kdpD of other bacteria, whereas a kdpE-like gene was absent in the vicinity of the two kdp operons. In response to K+ limitation (<0.05 mM external K+), only kdp2 (and not kdp1) expression could be detected as a 5.3-kb transcript on Northern blots, indicating that kdpA2B2G2C2 genes constitute a polycystronic operon. Unlike E. coli, addition of osmolytes like NaCl, or a change in pH of the medium did not enhance the kdp expression in Anabaena sp. strain L-31. Interestingly, the Anabaena sp. strain L-31 kdp2 operon was strongly induced in response to desiccation stress. The addition of K+ to K+ -starved cultures resulted in repression and degradation of kdp2 transcripts. Our results clearly show that kdp2 is the major kdp operon expressed in Anabaena sp. strain L-31 and may play an important role in adaptation to K+ limitation and desiccation stress.