Henk Bolhuis

The genome of the square archaeon Haloquadratum walsbyi : life at the limits of water activity

BackgroundThe square halophilic archaeon Haloquadratum walsbyi dominates NaCl-saturated and MgCl2 enriched aquatic ecosystems, which imposes a serious desiccation stress, caused by the extremely low water activity. The genome sequence was analyzed and physiological and physical experiments were carried out in order to reveal how H. walsbyi has specialized into its narrow and hostile ecological niche and found ways to cope with the desiccation stress.ResultsA rich repertoire of proteins involved in phosphate metabolism, phototrophic growth and extracellular protective polymers, including the largest archaeal protein (9159 amino acids), a homolog to eukaryotic mucins, are amongst the most outstanding features. A relatively low GC content (47.9%), 15–20% less than in other halophilic archaea, and one of the lowest coding densities (76.5%) known for prokaryotes might be an indication for the specialization in its unique environmentConclusionAlthough no direct genetic indication was found that can explain how this peculiar organism retains its square shape, the genome revealed several unique adaptive traits that allow this organism to thrive in its specific and extreme niche.

Multidrug resistance in Lactococcus lactis : Evidence for ATP-dependent drug extrusion from the inner leaflet of the cytoplasmic membrane

Lactococcus lactis possesses an ATP‐dependent drug extrusion system which shares functional properties with the mammalian multidrug resistance (MDR) transporter P‐glycoprotein. One of the intriguing aspects of both transporters is their ability to interact with a broad range of structurally unrelated amphiphilic compounds. It has been suggested that P‐glycoprotein removes drugs directly from the membrane. Evidence is presented that this model is correct for the lactococcal multidrug transporter through studies of the extrusion mechanism of BCECF‐AM and cationic diphenylhexatriene (DPH) derivatives from the membrane. The non‐fluorescent probe BCECF‐AM can be converted intracellularly into its fluorescent derivative, BCECF, by non‐specific esterase activities. The development of fluorescence was decreased upon energization of the cells. These and kinetic studies showed that BCECF‐AM is actively extruded from the membrane before it can be hydrolysed intracellularly. The increase in fluorescence intensity due to the distribution of TMA‐DPH into the phospholipid bilayer is a biphasic process. This behaviour reflects the fast entry of TMA‐DPH into the outer leaflet followed by a slower transbilayer movement to the inner leaflet of the membrane. The initial rate of TMA‐DPH extrusion correlates with the amount of probe associated with the inner leaflet. Taken together, these results demonstrate that the lactococcal MDR transporter functions as a ‘hydrophobic vacuum cleaner’, expelling drugs from the inner leaflet of the lipid bilayer. Thus, the ability of amphiphilic substrates to partition in the inner leaflet of the membrane is a prerequisite for recognition by multidrug transporters.

The Lactococcal lmrP Gene Encodes a Proton Motive Force- dependent Drug Transporter*

To genetically dissect the drug extrusion systems of Lactococcus lactis, a chromosomal DNA library was made in Escherichia coli and recombinant strains were selected for resistance to high concentrations of ethidium bromide. Recombinant strains were found to be resistant not only to ethidium bromide but also to daunomycin and tetraphenylphosphonium. The drug resistance is conferred by the lmrP gene, which encodes a hydrophobic polypeptide of 408 amino acid residues with 12 putative membrane-spanning segments. Some sequence elements in this novel membrane protein share similarity to regions in the transposon Tn10-encoded tetracycline resistance determinant TetA, the multidrug transporter Bmr from Bacillus subtilis, and the bicyclomycin resistance determinant Bcr from E. coli. Drug resistance associated with lmrP expression correlated with energy-dependent extrusion of the molecules. Drug extrusion was inhibited by ionophores that dissipate the proton motive force but not by the ATPase inhibitor ortho-vanadate. These observations are indicative for a drug-proton antiport system. A lmrP deletion mutant was constructed via homologous recombination using DNA fragments of the flanking region of the gene. The L. lactis (ΔlmrP) strain exhibited residual ethidium extrusion activity, which in contrast to the parent strain was inhibited by ortho-vanadate. The results indicate that in the absence of the functional drug-proton antiporter LmrP, L. lactis is able to overexpress another, ATP-dependent, drug extrusion system. These findings substantiate earlier studies on the isolation and characterization of drug-resistant mutants of L. lactis (Bolhuis, H., Molenaar, D., Poelarends, G., van Veen, H. W., Poolman, B., Driessen, A. J. M., and Konings, W. N.(1994) J. Bacteriol. 176, 6957-6964).

Analysis of bacterial and archaeal diversity in coastal microbial mats using massive parallel 16S rRNA gene tag sequencing.

Coastal microbial mats are small-scale and largely closed ecosystems in which a plethora of different functional groups of microorganisms are responsible for the biogeochemical cycling of the elements. Coastal microbial mats play an important role in coastal protection and morphodynamics through stabilization of the sediments and by initiating the development of salt-marshes. Little is known about the bacterial and especially archaeal diversity and how it contributes to the ecological functioning of coastal microbial mats. Here, we analyzed three different types of coastal microbial mats that are located along a tidal gradient and can be characterized as marine (ST2), brackish (ST3) and freshwater (ST3) systems. The mats were sampled during three different seasons and subjected to massive parallel tag sequencing of the V6 region of the 16S rRNA genes of Bacteria and Archaea. Sequence analysis revealed that the mats are among the most diverse marine ecosystems studied so far and consist of several novel taxonomic levels ranging from classes to species. The diversity between the different mat types was far more pronounced than the changes between the different seasons at one location. The archaeal community for these mats have not been studied before and revealed a strong reaction on a short period of draught during summer resulting in a massive increase in halobacterial sequences, whereas the bacterial community was barely affected. We concluded that the community composition and the microbial diversity were intrinsic of the mat type and depend on the location along the tidal gradient indicating a relation with salinity.

The role of transport processes in survival of lactic acid bacteria. Energy transduction and multidrug resistance

Lactic acid bacteria play an essential role in many food fermentation processes. They are anaerobic organisms which obtain their metabolic energy by substrate phosphorylation. In addition three secondary energy transducing processes can contribute to the generation of a proton motive force: proton/substrate symport as in lactic acid excretion, electrogenic precursor/product exchange as in malolactic and citrolactic fermentation and histidine/histamine exchange, and electrogenic uniport as in malate and citrate uptake in Leuconostoc oenos. In several of these processes additional H+ consumption occurs during metabolism leading to the generation of a pH gradient, internally alkaline. Lactic acid bacteria have also developed multidrug resistance systems. In Lactococcus lactis three toxin excretion systems have been characterized: cationic toxins can be excreted by a toxin/proton antiport system and by an ABC-transporter. This cationic ABC-transporter has surprisingly high structural an d functional analogy with the human MDR1-(P-glycoprotein). For anions an ATP-driven ABC-like excretion systems exist.

Horizontal transfer of the nitrogen fixation gene cluster in the cyanobacterium Microcoleus chthonoplastes

The filamentous, non-heterocystous cyanobacterium Microcoleus chthonoplastes is a cosmopolitan organism, known to build microbial mats in a variety of different environments. Although most of these cyanobacterial mats are known for their capacity to fix dinitrogen, M. chthonoplastes has not been assigned as a diazotrophic organism. None of the strains that were correctly identified as M. chthonoplastes has been shown to fix dinitrogen and it has repeatedly been reported that these organisms lacked the cyanobacterial nifH, the structural gene for dinitrogenase reductase. In this study, we show that a complete nif-gene cluster is present in the genome of M. chthonoplastes PCC 7420 and that the three structural nitrogenase genes, nifHDK, are present in a collection of axenic strains of M. chthonoplastes from distant locations. Phylogenetic analysis of nifHDK revealed that they cluster with the Deltaproteobacteria and that they are closely related to Desulfovibrio. The nif operon is flanked by typical cyanobacterial genes, suggesting that it is an integral part of the M. chthonoplastes genome. In this study, we provide evidence that the nif operon of M. chthonoplastes is acquired through horizontal gene transfer. Moreover, the presence of the same nif-cluster in M. chthonoplastes isolates derived from various sites around the world suggests that this horizontal gene transfer event must have occurred early in the evolution of M. chthonoplastes. We have been unable to express nitrogenase in cultures of M. chthonoplastes, but we show that these genes were expressed under natural conditions in the field.

Insights into the Physiology and Ecology of the Brackish-Water-Adapted Cyanobacterium Nodularia spumigena CCY9414 Based on a Genome-Transcriptome Analysis

Nodularia spumigena is a filamentous diazotrophic cyanobacterium that dominates the annual late summer cyanobacterial blooms in the Baltic Sea. But N. spumigena also is common in brackish water bodies worldwide, suggesting special adaptation allowing it to thrive at moderate salinities. A draft genome analysis of N. spumigena sp. CCY9414 yielded a single scaffold of 5,462,271 nucleotides in length on which genes for 5,294 proteins were annotated. A subsequent strand-specific transcriptome analysis identified more than 6,000 putative transcriptional start sites (TSS). Orphan TSSs located in intergenic regions led us to predict 764 non-coding RNAs, among them 70 copies of a possible retrotransposon and several potential RNA regulators, some of which are also present in other N2-fixing cyanobacteria. Approximately 4% of the total coding capacity is devoted to the production of secondary metabolites, among them the potent hepatotoxin nodularin, the linear spumigin and the cyclic nodulapeptin. The transcriptional complexity associated with genes involved in nitrogen fixation and heterocyst differentiation is considerably smaller compared to other Nostocales. In contrast, sophisticated systems exist for the uptake and assimilation of iron and phosphorus compounds, for the synthesis of compatible solutes, and for the formation of gas vesicles, required for the active control of buoyancy. Hence, the annotation and interpretation of this sequence provides a vast array of clues into the genomic underpinnings of the physiology of this cyanobacterium and indicates in particular a competitive edge of N. spumigena in nutrient-limited brackish water ecosystems.

The gene for a halophilic beta-galactosidase (bgaH) of Haloferax alicantei as a reporter gene for promoter analyses in Halobacterium salinarum.

Investigations of transcriptional regulation and the characterization of promoters in homologous expression systems are most easily performed using suitable reporter genes. Presumably because of the high internal salt concentration in halophilic Archaea, the successful application of the commonly used reporter genes has not been reported so far. Recently, the gene for an extremely halophilic β‐galactosidase (bgaH) from Haloferax alicantei has become available. After transformation of Halobacterium salinarum with a vector‐carrying bgaH, the enzyme activity in cell lysates could be readily determined by a simple colorimetric assay and colonies could be screened for activity on plates containing Xgal substrate. Expression of bgaH under the control of various halobacterial promoters of known strength led to different specific β‐galactosidase activities in the lysates. Using Northern blot hybridization and semiquantitative RT‐PCR, it was shown that the bgaH transcript level corresponded to the specific enzyme activity. Therefore, the bgaH gene of Haloferax alicantei appears to be a useful tool for in vivo studies of gene expression in Halobacterium salinarum and possibly other halophilic Archaea.

Actinomycete integrative and conjugative elements

This paper reviews current knowledge on actinomycete integrative and conjugative elements (AICEs). The best characterised AICEs, pSAM2 of Streptomyces ambofaciens (10.9 kb), SLP1 (17.3 kb) of Streptomyces coelicolor and pMEA300 of Amycolatopsis methanolica (13.3 kb), are present as integrative elements in specific tRNA genes, and are capable of conjugative transfer. These AICEs have a highly conserved structural organisation, with functional modules for excision/integration, replication, conjugative transfer, and regulation. Recently, it has been shown that pMEA300 and the related elements pMEA100 of Amycolatopsis mediterranei and pSE211 of Saccharopolyspora erythraea form a novel group of AICEs, the pMEA-elements, based on the unique characteristics of their replication initiator protein RepAM. Evaluation of a large collection of Amycolatopsis isolates has allowed identification of multiple pMEA-like elements. Our data show that, as AICEs, they mainly coevolved with their natural host in an integrated form, rather than being dispersed via horizontal gene transfer. The pMEA-like elements could be separated into two distinct populations from different geographical origins. One group was most closely related to pMEA300 and was found in isolates from Australia and Asia and pMEA100-related sequences were present in European isolates. Genome sequence data have enormously contributed to the recent insight that AICEs are present in many actinomycete genera. The sequence data also provide more insight into their evolutionary relationships, revealing their modular composition and their likely combined descent from bacterial plasmids and bacteriophages. Evidence is accumulating that AICEs act as modulators of host genome diversity and are also involved in the acquisition of secondary metabolite clusters and foreign DNA via horizontal gene transfer. Although still speculative, these AICEs may play a role in the spread of antibiotic resistance factors into pathogenic bacteria. The novel insights on AICE characteristics presented in this review may be used for the effective construction of new vectors that allows us to engineer and optimise strains for the production of commercially and medically interesting secondary metabolites, and bioactive proteins.

Coastal microbial mat diversity along a natural salinity gradient.

The North Sea coast of the Dutch barrier island of Schiermonnikoog is covered by microbial mats that initiate a succession of plant communities that eventually results in the development of a densely vegetated salt marsh. The North Sea beach has a natural elevation running from the low water mark to the dunes resulting in gradients of environmental factors perpendicular to the beach. These gradients are due to the input of seawater at the low water mark and of freshwater from upwelling groundwater at the dunes and rainfall. The result is a natural and dynamic salinity gradient depending on the tide, rainfall and wind. We studied the microbial community composition in thirty three samples taken every ten meters along this natural salinity gradient by using denaturing gradient gel electrophoresis (DGGE) of rRNA gene fragments. We looked at representatives from each Domain of life (Bacteria, Archaea and Eukarya) and with a particular emphasis on Cyanobacteria. Analysis of the DGGE fingerprints together with pigment composition revealed three distinct microbial mat communities, a marine community dominated by diatoms as primary producers, an intermediate brackish community dominated by Cyanobacteria as primary producers and a freshwater community with Cyanobacteria and freshwater green algae.