Genotype delimitation in the Nod-independent model legume Aeschynomene evenia

Research on the nitrogen-fixing symbiosis has been so far focused on two model legumes, Medicago truncatula and Lotus japonicus, which use a sophisticated infection process involving infection thread formation. However, in 25% of the legumes, the bacterial entry occurs more simply in an intercellular fashion. Among them, some semi-aquatic Aeschynomene species present the distinctive feature to form nitrogen-fixing nodules on both roots and stems following elicitation by photosynthetic bradyrhizobia that do not produce Nod factors. This interaction is believed to represent a living testimony of the ancestral state of the rhizobium-legume symbiosis. To decipher the molecular mechanisms of this unique Nod-independent nitrogen-fixing symbiosis, we previously identified A. evenia C. Wright as an appropriate model legume, because it displays all the requisites for molecular and genetic approaches. To advance the use of this new model legume species, here we characterized the intraspecific diversity found in A. evenia. For this, the accessions available in germplasm banks were collected and subjected to morphological investigations, genotyping with RAPD and SSR markers, molecular phylogenies using ITS and single nuclear gene sequences, and cross-compatibility tests. These combined analyses revealed an important intraspecific differentiation that led us to propose a new taxonomic classification for A. evenia comprising two subspecies and four varieties. The A. evenia ssp. evenia contains var. evenia and var. pauciciliata whereas A. evenia ssp. serrulata comprises var. serrulata and var. major. This study provides information to exploit efficiently the diversity encountered in A. evenia and proposes subsp. evenia as the most appropriate subspecies for future projects aimed at identifying plant determinants of the Nod-independent symbiotic process.

Photosynthetic Bradyrhizobium sp. strain ORS285 is capable of forming nitrogen-fixing root nodules on soybeans (Glycine max)

The ability of photosynthetic Bradyrhizobium strains ORS285 and ORS278 to nodulate soybeans was investigated. While the nod gene-deficient ORS278 strain induced bumps only on soybean roots, the nod gene-containing ORS285 strain formed nitrogen-fixing nodules. However, symbiotic efficiencies differed drastically depending on both the soybean genotype used and the culture conditions tested.

Aeschynomene evenia, a model plant for studying the molecular genetics of the nod-independent rhizobium-legume symbiosis

Research on the nitrogen-fixing symbiosis has been focused, thus far, on two model legumes, Medicago truncatula and Lotus japonicus, which use a sophisticated infection process involving infection thread formation. However, in 25% of the legumes, the bacterial entry occurs more simply in an intercellular fashion. Among them, some Aeschynomene spp. are nodulated by photosynthetic Bradyrhizobium spp. that do not produce Nod factors. This interaction is believed to represent a living testimony of the ancestral state of the rhizobium-legume symbiosis. To decipher the mechanisms of this Nod-independent process, we propose Aeschynomene evenia as a model legume because it presents all the characteristics required for genetic and molecular analysis. It is a short-perennial and autogamous species, with a diploid and relatively small genome (2n=20; 460 Mb/1C). A. evenia 'IRFL6945' is nodulated by the well-characterized photosynthetic Bradyrhizobium sp. strain ORS278 and is efficiently transformed by Agrobacterium rhizogenes. Aeschynomene evenia is genetically homozygous but polymorphic accessions were found. A manual hybridization procedure has been set up, allowing directed crosses. Therefore, it should be relatively straightforward to unravel the molecular determinants of the Nod-independent process in A. evenia. This should shed new light on the evolution of rhizobium-legume symbiosis and could have important agronomic implications.

Comparative genomics of aeschynomene symbionts: insights into the ecological lifestyle of nod-independent photosynthetic bradyrhizobia

Tropical aquatic species of the legume genus Aeschynomene are stem- and root-nodulated by bradyrhizobia strains that exhibit atypical features such as photosynthetic capacities or the use of a nod gene-dependent (ND) or a nod gene-independent (NI) pathway to enter into symbiosis with legumes. In this study we used a comparative genomics approach on nine Aeschynomene symbionts representative of their phylogenetic diversity. We produced draft genomes of bradyrhizobial strains representing different phenotypes: five NI photosynthetic strains (STM3809, ORS375, STM3847, STM4509 and STM4523) in addition to the previously sequenced ORS278 and BTAi1 genomes, one photosynthetic strain ORS285 hosting both ND and NI symbiotic systems, and one NI non-photosynthetic strain (STM3843). Comparative genomics allowed us to infer the core, pan and dispensable genomes of Aeschynomene bradyrhizobia, and to detect specific genes and their location in Genomic Islands (GI). Specific gene sets linked to photosynthetic and NI/ND abilities were identified, and are currently being studied in functional analyses.

Photosynthetic Bradyrhizobium Sp. strain ORS285 synthesizes 2-O-methylfucosylated lipochitooligosaccharides for nod gene-dependent interaction with Aeschynomene plants

Bradyrhizobium sp. strain ORS285 is a photosynthetic bacterium that forms nitrogen-fixing nodules on the roots and stems of tropical aquatic legumes of the Aeschynomene genus. The symbiotic interaction of Bradyrhizobium sp. strain ORS285 with certain Aeschynomene spp. depends on the presence of nodulation (nod) genes whereas the interaction with other species is nod gene independent. To study the nod gene-dependent molecular dialogue between Bradyrhizobium sp. strain ORS285 and Aeschynomene spp., we used a nodB-lacZ reporter strain to monitor the nod gene expression with various flavonoids. The flavanones liquiritigenin and naringenin were found to be the strongest inducers of nod gene expression. Chemical analysis of the culture supernatant of cells grown in the presence of naringenin showed that the major Nod factor produced by Bradyrhizobium sp. strain ORS285 is a modified chitin pentasaccharide molecule with a terminal N-C(18:1)-glucosamine and with a 2-O-methyl fucose linked to C-6 of the reducing glucosamine. In this respect, the Bradyrhizobium sp. strain ORS285 Nod factor is the same as the major Nod factor produced by the nonphotosynthetic Bradyrhizobium japonicum USDA110 that nodulates the roots of soybean. This suggests a classic nod gene-dependent molecular dialogue between Bradyrhizobium sp. strain ORS285 and certain Aeschynomene spp. This is supported by the fact that B. japonicum USDA110 is able to form N(2)-fixing nodules on both the roots and stems of Aeschynomene afraspera.

Nodulation of Aeschynomene afraspera and A. indica by photosynthetic Bradyrhizobium Sp. strain ORS285: the nod-dependent versus the nod-independent symbiotic interaction

Here, we present a comparative analysis of the nodulation processes of Aeschynomene afraspera and A. indica that differ in their requirement for Nod factors (NF) to initiate symbiosis with photosynthetic bradyrhizobia. The infection process and nodule organogenesis was examined using the green fluorescent protein-labeled Bradyrhizobium sp. strain ORS285 able to nodulate both species. In A. indica, when the NF-independent strategy is used, bacteria penetrated the root intercellularly between axillary root hairs and invaded the subepidermal cortical cells by invagination of the host cell wall. Whereas the first infected cortical cells collapsed, the infected ones immediately beneath kept their integrity and divided repeatedly to form the nodule. In A. afraspera, when the NF-dependent strategy is used, bacteria entered the plant through epidermal fissures generated by the emergence of lateral roots and spread deeper intercellularly in the root cortex, infecting some cortical cells during their progression. Whereas the infected cells of the lower cortical layers divided rapidly to form the nodule, the infected cells of the upper layers gave rise to an outgrowth in which the bacteria remained enclosed in large tubular structures. Together, two distinct modes of infection and nodule organogenesis coexist in Aeschynomene legumes, each displaying original features.

Bacterial RuBisCO is required for efficient Bradyrhizobium/Aeschynomene symbiosis

Rhizobia and legume plants establish symbiotic associations resulting in the formation of organs specialized in nitrogen fixation. In such organs, termed nodules, bacteria differentiate into bacteroids which convert atmospheric nitrogen and supply the plant with organic nitrogen. As a counterpart, bacteroids receive carbon substrates from the plant. This rather simple model of metabolite exchange underlies symbiosis but does not describe the complexity of bacteroids' central metabolism. A previous study using the tropical symbiotic model Aeschynomene indica/photosynthetic Bradyrhizobium sp. ORS278 suggested a role of the bacterial Calvin cycle during the symbiotic process. Herein we investigated the role of two RuBisCO gene clusters of Bradyrhizobium sp. ORS278 during symbiosis. Using gene reporter fusion strains, we showed that cbbL1 but not the paralogous cbbL2 is expressed during symbiosis. Congruently, CbbL1 was detected in bacteroids by proteome analysis. The importance of CbbL1 for symbiotic nitrogen fixation was proven by a reverse genetic approach. Interestingly, despite its symbiotic nitrogen fixation defect, the cbbL1 mutant was not affected in nitrogen fixation activity under free living state. This study demonstrates a critical role for bacterial RuBisCO during a rhizobia/legume symbiotic interaction.

Nodulation of Crotalaria podocarpa DC. by Methylobacterium nodulans displays very unusual features

Crotalaria are plants of the Fabaceae family whose nodulation characteristics have been little explored despite the recent discovery of their unexpected ability to be efficiently nodulated in symbiosis with bacteria of the genus Methylobacterium. It has been shown that methylotrophy plays a key role in this unusual symbiotic system, as it is expressed within the nodule and as non-methylotroph mutants had a depleting effect on plant growth response. Within the nodule, Methylobacterium is thus able to obtain carbon both from host plant photosynthesis and from methylotrophy. In this context, the aim of the present study was to show the histological and cytological impacts of both symbiotic and methylotrophic metabolism within Crotalaria podocarpa nodules. It was established that if Crotalaria nodules are multilobed, each lobe has the morphology of indeterminate nodules but with a different anatomy; that is, without root hair infection or infection threads. In the fixation zone, bacteroids display a spherical shape and there is no uninfected cell. Crotalaria nodulation by Methylobacterium displayed some very unusual characteristics such as starch storage within bacteroid-filled cells of the fixation zone and also the complete lysis of apical nodular tissues (where bacteria have a free-living shape and express methylotrophy). This lysis could possibly reflect the bacterial degradation of plant wall pectins through bacterial pectin methyl esterases, thus producing methanol as a substrate, allowing bacterial multiplication before release from the nodule.

The Nod factor-independent symbiotic signaling pathway: development of Agrobacterium rhizogenes-mediated transformation for the legume Aeschynomene indica

The nitrogen-fixing symbiosis between Aeschynomene indica and photosynthetic bradyrhizobia is the only legume-rhizobium association described to date that does not require lipochito-oligosaccharide Nod factors (NF). To assist in deciphering the molecular basis of this NF-independent interaction, we have developed a protocol for Agrobacterium rhizogenes-mediated transformation of A. indica. The cotransformation frequency (79%), the nodulation efficiency of transgenic roots (90%), and the expression pattern of the 35S Cauliflower mosaic virus promoter in transgenic nodules were all comparable to those obtained for model legumes. We have made use of this tool to monitor the heterologous spatio-temporal expression of the pMtENOD11-β-glucuronidase fusion, a widely used molecular reporter for rhizobial infection and nodulation in both legumes and actinorhizal plants. While MtENOD11 promoter activation was not observed in A. indica roots prior to nodulation, strong reporter-gene expression was observed in the invaded cells of young nodules and in the cell layers bordering the central zone of older nodules. We conclude that pMtENOD11 expression can be used as an infection-related marker in A. indica and that Agrobacterium rhizogenes-mediated root transformation of Aeschynomene spp. will be an invaluable tool for determining the molecular basis of the NF-independent symbiosis.

Degradation of Raw Starch by a Wild Amylolytic Strain of Lactobacillus plantarum

Lactobacillus plantarum A6, isolated from fermented cassava, can break down cassava raw starch that has not been subjected to preliminary physicochemical treatment. When the pH was kept at 6, the microorganism cultured in a bioreactor excreted a high alpha-amylase activity (60 U/ml). Synthesis of the enzyme occurred during the stationary phase and resulted in full hydrolysis of the cassava starch granules. This gave 41 g of lactic acid from 45 g of raw starch after 3 days of fermentation. Enzymatic attack was evident under scanning electron microscopy in the rougher appearance of the surface of starch granules and in the presence of large cavities in some of them. In contrast, when the pH was not regulated, only a small amount of alpha-amylase activity was produced (2 U/ml) and no decrease in the starch content of the medium was observed. However, under scanning electron microscopy, some granules displayed a rougher surface, which might have been the result of weak enzymatic attack.

Large-scale transposon mutagenesis of photosynthetic Bradyrhizobium sp. strain ORS278 reveals new genetic loci putatively important for nod-independent symbiosis with Aeschynomene indica

Photosynthetic Bradyrhizobium strains possess the unusual ability to form nitrogen-fixing nodules on a specific group of legumes in the absence of Nod factors. To obtain insight into the bacterial genes involved in this Nod-independent symbiosis, we screened 15,648 Tn5 mutants of Bradyrhizobium sp. strain ORS278 for clones affected in root symbiosis with Aeschynomene indica. From the 268 isolated mutants, 120 mutants were altered in nodule development (Ndv(-)) and 148 mutants were found to be deficient in nitrogen fixation (Fix(-)). More than 50% of the Ndv(-) mutants were found to be altered in purine biosynthesis, strengthening the previous hypothesis of a symbiotic role of a bacterial purine derivative during the Nod-independent symbiosis. The other Ndv(-) mutants were auxotrophic for pyrimidines and amino acids (leucine, glutamate, and lysine) or impaired in genes encoding proteins of unknown function. The Fix(-) mutants were found to be affected in a wide variety of cellular processes, including both novel (n = 56) and previously identified (n = 31) genes important in symbiosis. Among the novel genes identified, several were involved in the Calvin cycle, suggesting that CO(2) fixation could play an important role during this symbiosis.

Establishing nitrogen-fixing symbiosis with legumes: how many rhizobium recipes?

Rhizobia are phylogenetically disparate alpha- and beta-proteobacteria that have achieved the environmentally essential function of fixing atmospheric nitrogen (N(2)) in symbiosis with legumes. All rhizobia elicit the formation of root - or occasionally stem - nodules, plant organs dedicated to the fixation and assimilation of nitrogen. Bacterial colonization of these nodules culminates in a remarkable case of sustained intracellular infection in plants. Rhizobial phylogenetic diversity raised the question of whether these soil bacteria shared a common core of symbiotic genes. In this article, we review the cumulative evidence from recent genomic and genetic analyses pointing toward an unexpected variety of mechanisms that lead to symbiosis with legumes.

Identification of novel genes putatively involved in the photosystem synthesis of Bradyrhizobium sp. ORS 278

In aerobic anoxygenic phototrophs, oxygen is required for both the formation of the photosynthetic apparatus and an efficient cyclic electron transfer. Mutants of Bradyrhizobium sp. ORS278 affected in photosystem synthesis were selected by a bacteriochlorophyll fluorescence-based screening. Out of the 9,600 mutants of a random Tn5 insertion library, 50 clones, corresponding to insertions in 28 different genes, present a difference in fluorescence intensity compared to the WT. Besides enzymes and regulators known to be involved in photosystem synthesis, 14 novel components of the photosynthesis control are identified. Among them, two genes, hsIU and hsIV, encode components of a protein degradation complex, probably linked to the renewal of photosystem, an important issue in Bradyrhizobia which have to deal with harmful reactive oxygen species. The presence of homologs in non-photosynthetic bacteria for most of the regulatory genes identified during study suggests that they could be global regulators, as the RegA-RegB system.

Bacteriophytochromes in anoxygenic photosynthetic bacteria

Since the first discovery of a bacteriophytochrome in Rhodospirillum centenum, numerous bacteriophytochromes have been identified and characterized in other anoxygenic photosynthetic bacteria. This review is focused on the biochemical and biophysical properties of bacteriophytochromes with a special emphasis on their roles in the synthesis of the photosynthetic apparatus.

Antimicrobial resistance survey in a river receiving effluents from freshwater fish farms

AIMS

The aims of this study were: (i) to determine the proportions of Aeromonas spp. resistant to florfenicol (FC), oxolinic acid (OA) and oxytetracycline (OTC) along a river receiving effluents from fish farms, and (ii) to assess the relevance of using this bacterial group as an indicator for studying the consequences of the use and release of these aquacultural antimicrobials in the freshwater environment, as compared with performing antimicrobial measurements in sediments.

METHODS AND RESULTS

Sediment interstitial waters sampled along a river during two distinct climatic seasons were plated on an Aeromonas-selective medium supplemented or not with OA, OTC or FC. The October 2004 campaign showed an enrichment of OA- and OTC-resistant Aeromonas immediately downstream of the fish farms and a wastewater treatment plant. Two fish farms showed similar results in March 2005. In contrast, only 10 FC-resistant Aeromonas strains could be isolated, which revealed that minimum inhibitory concentrations of FC were greater than 64 microg ml(-1) and multiple antimicrobial resistances. Contamination of sediments by antimicrobials was detected but was not always co-localized with resistance peaks or known point sources of contamination.

CONCLUSIONS

Aeromonas could be valuable indicators of OA, OTC and FC resistance in the freshwater environment. Fish farms contribute to the contamination of the river by antimicrobials and resistant bacteria.

SIGNIFICANCE AND IMPACT OF THE STUDY

Considering the still very low proportion of FC-resistant Aeromonas, this study can be considered as a reference for further studies about this recently introduced veterinary antimicrobial agent.

Evolution of a bacteriophytochrome from light to redox sensor

Bacteriophytochromes are red/far-red photoreceptors that bacteria use to mediate sensory responses to their light environment. Here, we show that the photosynthetic bacterium Rhodopseudomonas palustris has two distinct types of bacteriophytochrome-related protein (RpBphP4) depending upon the strain considered. The first type binds the chromophore biliverdin and acts as a light-sensitive kinase, thus behaving as a bona fide bacteriophytochrome. However, in most strains, RpBphP4 does not to bind this chromophore. This loss of light sensing is replaced by a redox-sensing ability coupled to kinase activity. Phylogenetic analysis is consistent with an evolutionary scenario, where a bacteriophytochrome ancestor has adapted from light to redox sensing. Both types of RpBphP4 regulate the synthesis of light harvesting (LH2) complexes according to the light or redox conditions, respectively. They modulate the affinity of a transcription factor binding to the promoter regions of LH2 complex genes by controlling its phosphorylation status. This is the first complete description of a bacteriophytochrome signal transduction pathway involving a two-component system.

Legumes symbioses: absence of Nod genes in photosynthetic bradyrhizobia

Leguminous plants (such as peas and soybeans) and rhizobial soil bacteria are symbiotic partners that communicate through molecular signaling pathways, resulting in the formation of nodules on legume roots and occasionally stems that house nitrogen-fixing bacteria. Nodule formation has been assumed to be exclusively initiated by the binding of bacterial, host-specific lipochito-oligosaccharidic Nod factors, encoded by the nodABC genes, to kinase-like receptors of the plant. Here we show by complete genome sequencing of two symbiotic, photosynthetic, Bradyrhizobium strains, BTAi1 and ORS278, that canonical nodABC genes and typical lipochito-oligosaccharidic Nod factors are not required for symbiosis in some legumes. Mutational analyses indicated that these unique rhizobia use an alternative pathway to initiate symbioses, where a purine derivative may play a key role in triggering nodule formation.

A singular bacteriophytochrome acquired by lateral gene transfer

Bacteriophytochromes are phytochrome-like proteins that mediate photosensory responses in various bacteria according to their light environment. The genome of the photosynthetic and plant-symbiotic Bradyrhizobium sp. strain ORS278 revealed the presence of a genomic island acquired by lateral transfer harboring a bacteriophytochrome gene, BrBphP3.ORS278, and genes involved in the synthesis of phycocyanobilin and gas vesicles. The corresponding protein BrBphP3.ORS278 is phylogenetically distant from the other (bacterio)phytochromes described thus far and displays a series of unusual properties. It binds phycocyanobilin as a chromophore, a unique feature for a bacteriophytochrome. Moreover, its C-terminal region is short and displays no homology with any known functional domain. Its dark-adapted state absorbs maximally around 610 nm, an unusually short wavelength for (bacterio)phytochromes. This form is designated as Po for orange-absorbing form. Upon illumination, a photo-reversible switch occurs between the Po form and a red (670 nm)-absorbing form (Pr), which rapidly backreacts in the dark. Because of this instability, illumination results in a mixture of the Po and Pr states in proportions that depend on the intensity. These uncommon features suggest that BrBphP3.ORS278 could be fitted to measure light intensity rather than color.

Role of methylotrophy during symbiosis between Methylobacterium nodulans and Crotalaria podocarpa

Some rare leguminous plants of the genus Crotalaria are specifically nodulated by the methylotrophic bacterium Methylobacterium nodulans. In this study, the expression and role of bacterial methylotrophy were investigated during symbiosis between M. nodulans, strain ORS 2060T, and its host legume, Crotalaria podocarpa. Using lacZ fusion to the mxaF gene, we showed that the methylotroph genes are expressed in the root nodules, suggesting methylotrophic activity during symbiosis. In addition, loss of the bacterial methylotrophic function significantly affected plant development. Indeed, inoculation of M. nodulans nonmethylotroph mutants in C. podocarpa decreased the total root nodule number per plant up to 60%, decreased the whole-plant nitrogen fixation capacity up to 42%, and reduced the total dry plant biomass up to 46% compared with the wild-type strain. In contrast, inoculation of the legume C. podocarpa with nonmethylotrophic mutants complemented with functional mxa genes restored the symbiotic wild phenotype. These results demonstrate the key role of methylotrophy during symbiosis between M. nodulans and C. podocarpa.

PpsR: a multifaceted regulator of photosynthesis gene expression in purple bacteria

Purple bacteria control the level of expression and the composition of their photosystem according to light and redox conditions. This control involves several regulatory systems that have been now well characterized. Among them, the PpsR regulator plays a central role, because it directly or indirectly controls the synthesis of all of the different components of the photosystem. In this review, we report our knowledge of the PpsR protein, highlighting the diversity of its mode of action and focusing on the proteins identified in four model purple bacteria (Rhodobacter capsulatus, Rhodobacter sphaeroides, Rubrivivax gelatinosus, Bradyrhizobium ORS278). This regulator exhibits unique regulatory features in each bacterium: it can activate and/or repress the expression of photosynthesis genes, its activity can be modulated or not by the redox conditions, it can interact with other specific regulators and therefore be involved differently in light and/or redox regulatory circuits.

A new type of bacteriophytochrome acts in tandem with a classical bacteriophytochrome to control the antennae synthesis in Rhodopseudomonas palustris

Phytochromes are chromoproteins found in plants and bacteria that switch between two photointerconvertible forms via the photoisomerization of their chromophore. These two forms, Pr and Pfr, absorb red and far-red light, respectively. We have characterized the biophysical and biochemical properties of two bacteriophytochromes, RpBphP2 and RpBphP3, from the photosynthetic bacterium Rhodopseudomonas palustris. Their genes are contiguous and localized near the pucBAd genes encoding the polypeptides of the light harvesting complexes LH4, whose synthesis depends on the light intensity. At variance with all (bacterio)phytochromes studied so far, the light-induced isomerization of the chromophore of RpBphP3 converts the Pr form to a form absorbing at shorter wavelength around 645 nm, designated as Pnr for near red. The quantum yield for the transformation of Pr into Pnr is about 6-fold smaller than for the reverse reaction. Both RpBphP2 and RpBphP3 autophosphorylate in their dark-adapted Pr forms and transfer their phosphate to a common response regulator Rpa3017. Under semiaerobic conditions, LH4 complexes replace specifically the LH2 complexes in wild-type cells illuminated by wavelengths comprised between 680 and 730 nm. In contrast, mutants deleted in each of these two bacteriophytochromes display no variation in the composition of their light harvesting complexes whatever the light intensity. From both the peculiar properties of these bacteriophytochromes and the phenotypes of their deletion mutants, we propose that they operate in tandem to control the synthesis of LH4 complexes by measuring the relative intensities of 645 and 710 nm lights.

Methylobacterium nodulans sp. nov., for a group of aerobic, facultatively methylotrophic, legume root-nodule-forming and nitrogen-fixing bacteria

Data on 72 non-pigmented bacterial strains that specifically induce nitrogen-fixing root nodules on the legume species Crotalaria glaucoides, Crotalaria perrottetii and Crotalaria podocarpa are reviewed. By SDS-PAGE analysis of total protein patterns and by 16S rRNA PCR-RFLP, these strains form a homogeneous group that is separate from other legume root-nodule-forming bacteria. The 16S rRNA gene-based phylogeny indicates that these bacteria belong to the genus Methylobacterium. They can grow on C(1) compounds such as methanol, formate and formaldehyde but not methylamine as sole carbon source, and carry an mxaF gene, encoding methanol dehydrogenase, which supports their methylotrophic metabolism. Presence of a nodA nodulation gene, and ability to nodulate plants of Crotalaria species and to fix nitrogen are features that separate the strains currently included in this group from other members of the genus Methylobacterium. The present study includes additional genotypic and phenotypic characterization of this novel Methylobacterium species, i.e. nifH gene sequence, morphology, physiology, enzymic and carbon source assimilation tests and antibiotic resistance. The name Methylobacterium nodulans sp. nov. (type strain, ORS 2060(T)=CNCM I 2342(T)=LMG 21967(T)) is proposed for this group of root-nodule-forming bacteria.

Light and Redox Control of Photosynthesis Gene Expression in Bradyrhizobium

The two closely related bacteria Bradyrhizobium and Rhodopseudomonas palustris show an unusual mechanism of regulation of photosystem formation by light thanks to a bacteriophytochrome that antirepresses the regulator PpsR. In these two bacteria, we found out, unexpectedly, that two ppsR genes are present. We show that the two Bradyrhizobium PpsR proteins exert antagonistic effects in the regulation of photosystem formation with a classical repressor role for PpsR2 and an unexpected activator role for PpsR1. DNase I footprint analysis show that both PpsR bind to the same DNA TGTN12ACA motif that is present in tandem in the bchC promoter and the crtED intergenic region. Interestingly, the cycA and aerR promoter regions that contain only one conserved palindrome are recognized by PpsR2, but not PpsR1. Further biochemical analyses indicate that PpsR1 only is redox sensitive through the formation of an intermolecular disulfide bond, which changes its oligomerization state from a tetramer to an octamer under oxidizing conditions. Moreover, PpsR1 presents a higher DNA affinity under its reduced form in contrast to what has been previously found for PpsR or its homolog CrtJ from the Rhodobacter species. These results suggest that regulation of photosystem synthesis in Bradyrhizobium involves two PpsR competing for the binding to the same photosynthesis genes and this competition might be modulated by two factors: light via the antagonistic action of a bacteriophytochrome on PpsR2 and redox potential via the switch of PpsR1 oligomerization state.

Two Distinct crt Gene Clusters for Two Different Functional Classes of Carotenoid in Bradyrhizobium

Aerobic photosynthetic bacteria possess the unusual characteristic of producing different classes of carotenoids. In this study, we demonstrate the presence of two distinct crt gene clusters involved in the synthesis of spirilloxanthin and canthaxanthin in a Bradyrhizobium strain. Each cluster contains the genes crtE, crtB, and crtI leading to the common precursor lycopene. We show that spirilloxanthin is associated with the photosynthetic complexes, while canthaxanthin protects the bacteria from oxidative stress. Only the spirilloxanthin crt genes are regulated by light via the control of a bacteriophytochrome. Despite this difference in regulation, the biosyntheses of both carotenoids are strongly interconnected at the level of the common precursors. Phylogenetic analysis suggests that the canthaxanthin crt gene cluster has been acquired by a lateral gene transfer. This acquisition may constitute a major selective advantage for this class of bacteria, which photosynthesize only under conditions where harmful reactive oxygen species are generated.

Bacteriophytochrome and regulation of the synthesis of the photosynthetic apparatus in Rhodopseudomonas palustris: pitfalls of using laboratory strains

The synthesis of the photosynthetic apparatus of different strains of Rhodopseudomonas palustris has been studied as a function of the oxygen concentration and far-red light. For strain CEA001, only a small amount of photosynthetic apparatus is synthesized in the dark for oxygen concentration higher than 8% whereas synthesis is strongly enhanced by far-red light illumination. This enhancement is due to the action of a bacteriophytochrome (ORF2127/ORF2128), which antagonizes the repressor PpsR. On the contrary, a large fraction of photosystem is synthesized in the dark and far-red illumination induces no enhancement in strain CGA009. This difference in phenotype of strain CGA009 is explained by a single point-mutation R428C in the helix-turn-helix DNA binding motif of PpsR, rendering it inactive. In addition, a frame-shift mutation had occurred in the gene encoding bacteriophytochrome (ORF2127/ORF2128), conducting to a truncated inactive sensor. We propose that these mutations occurred in culture. Bacteria have developed a sophisticated regulatory process to synthesize their photosynthetic apparatus when light is available. This process is a critical advantage for the bacteria under natural conditions since they optimize their development depending on the available energy resources. On the contrary, under laboratory growth conditions where there is no substrate limitation, there is no crucial need for such a regulation and deleterious mutations affecting this process are of no importance.

Nitrogen-fixing symbiosis between photosynthetic bacteria and legumes

Rhizobia having photosynthetic systems form nitrogen-fixing nodules on the stem and/or root of some species of the legumes Aeschynomene and Lotononis. This review is focused on the recent knowledge about the physiology, genetics and role of the photosystem in these bacteria. Photosynthetic electron transport seems to involve reaction centers, soluble cytochrome c2 and cytochrome bc1. Anaerobically, the electron transport system becomes over-reduced. The photosynthesis genes have been partially characterized; their organization is classical but their regulation is unusual as it is activated by far-red light via a bacteriophytochrome. This original mechanism of regulation seems well adapted to promote photosynthesis during stem symbiosis. Photosynthesis plays a major role in the efficiency of stem nodulation. It is also observed that infrared light stimulates nitrogen fixation in nodules containing photosynthetic bacteroids, suggesting that photosynthesis may additionally provides energy for nitrogen fixation, allowing for more efficient plant growth. Other aspects of these bacteria are discussed, in particular their taxonomic position and nodulation ability, the role of carotenoids and the potential for application of photosynthetic rhizobia in rice culture.

Phenotypic and genotypic diversity of rhizobia nodulating Pterocarpus erinaceus and P. lucens in Senegal

A total of fifty root nodules isolates of fast-growing and slow growing rhizobia from Pterocarpus ennaceus and Pterocarpus lucens respectively native of sudanean and sahelian regions of Senegal were characterized. These isolates were compared to representative strains of known rhizobial species. Twenty-two new isolates were slow growers and twenty-eight were fast growers. A polyphasic approach was performed including comparative total protein sodium dodecyl sulphate polyacrylamide gel (SDS-PAGE) profile analysis; 16S rDNA and 16S-23S rDNA intergenic spacer (IGS) sequence analysis. By SDS-PAGE the slow growing isolates grouped in one major cluster containing reference strains of Bradyrhizobium sp. including strains isolated in Africa, in Brazil and in New Zealand. Most of the fast-growing rhizobia grouped in four different clusters or were separate strains related to Rhizobium and Mesorhizobium strains. The 16S rDNA and 16S-23S rDNA IGS sequences analysis showed accurately the differentiation of fast growing rhizobia among the Rhizobium and Mesorbizobium genospecies. The representative strains of slow growing rhizobia were identified as closely related to Bradyrbizobium elkanii and Bradyrhizobium japonicum. Based on 16S rDNA sequence analysis, one slow growing strain (ORS199) was phylogenetically related to Bradyrbizobium sp. (Lupinus) and Blastobacter denitrificans. This position of ORS 199 was not confirmed by IGS sequence divergence. We found no clear relation between the diversity of strains, the host plants and the ecogeographical origins.

Bacteriophytochrome controls photosystem synthesis in anoxygenic bacteria

Plants use a set of light sensors to control their growth and development in response to changes in ambient light. In particular, phytochromes exert their regulatory activity by switching between a biologically inactive red-light-absorbing form (Pr) and an active far-red-light absorbing form (Pfr). Recently, biochemical and genetic studies have demonstrated the occurrence of phytochrome-like proteins in photosynthetic and non-photosynthetic bacteria--but little is known about their functions. Here we report the discovery of a bacteriophytochrome located downstream from the photosynthesis gene cluster in a Bradyrhizobium strain symbiont of Aeschynomene. The synthesis of the complete photosynthetic apparatus is totally under the control of this bacteriophytochrome. A similar behaviour is observed for the closely related species Rhodopseudomonas palustris, but not for the more distant anoxygenic photosynthetic bacteria of the genus Rhodobacter, Rubrivivax or Rhodospirillum. Unlike other (bacterio)phytochromes, the carboxy-terminal domain of this bacteriophytochrome contains no histidine kinase features. This suggests a light signalling pathway involving direct protein-protein interaction with no phosphorelay cascade. This specific mechanism of regulation may represent an important ecological adaptation to optimize the plant-bacteria interaction.

[ Nodulation of certain legumes of the genus Crotalaria by the new species Methylobacterium]

We studied a collection of 126 rhizobial isolates from eight species of Crotalaria (C. comosa, C. glaucoides, C. goreensis, C. hyssopifolia, C. lathyroides, C. perrottetii, C. podocarpa, and C. retusa) growing in Senegal. Nodulation and nitrogen-fixation tests on nine Crotalaria species revealed two specificity groups within the genus Crotalaria. Group I consists of plants solely nodulated by very specific fast-growing strains. Group II plants are nodulated by slow-growing strains similar to promiscuous Bradyrhizobium spp. strains already reported to nodulate many tropical legumes. SDS-PAGE studies showed that slow-growing strains grouped with Bradyrhizobium while fast-growing strains constituted a homogeneous group distinct from all known rhizobia. Amplified ribosomal DNA restriction analysis (ARDRA) of 10 representative strains of this group using four restriction enzymes showed a single pattern for each enzyme confirming the high homogeneity of group I. The 16S rDNA sequence analysis revealed that this specific group belonged to the genus Methylobacterium, thus constituting a new branch of nodulating bacteria.

Characterization of high-level fluoroquinolone resistance in Escherichia coli O78:K80 isolated from turkeys

Fluoroquinolone resistance was characterized in Escherichia coli O78:K80 isolated from diseased turkeys. The level of resistance to fluoroquinolones of the isolates appeared closely correlated with substitutions in GyrA and ParC, but not with the production of the AcrAB efflux pump. Among isolates highly resistant to ciprofloxacin (MIC 8 mg/L) and harbouring identical substitutions (two in GyrA and one in ParC), two close but distinguishable ribotypes were identified. This indicated that at least two independent selection events may have occurred.

Methylotrophic Methylobacterium bacteria nodulate and fix nitrogen in symbiosis with legumes

Rhizobia described so far belong to three distinct phylogenetic branches within the alpha-2 subclass of Proteobacteria. Here we report the discovery of a fourth rhizobial branch involving bacteria of the Methylobacterium genus. Rhizobia isolated from Crotalaria legumes were assigned to a new species, "Methylobacterium nodulans," within the Methylobacterium genus on the basis of 16S ribosomal DNA analyses. We demonstrated that these rhizobia facultatively grow on methanol, which is a characteristic of Methylobacterium spp. but a unique feature among rhizobia. Genes encoding two key enzymes of methylotrophy and nodulation, the mxaF gene, encoding the alpha subunit of the methanol dehydrogenase, and the nodA gene, encoding an acyltransferase involved in Nod factor biosynthesis, were sequenced for the type strain, ORS2060. Plant tests and nodA amplification assays showed that "M. nodulans" is the only nodulating Methylobacterium sp. identified so far. Phylogenetic sequence analysis showed that "M. nodulans" NodA is closely related to Bradyrhizobium NodA, suggesting that this gene was acquired by horizontal gene transfer.

Characterization of the common nodulation genes of the photosynthetic Bradyrhizobium sp. ORS285 reveals the presence of a new insertion sequence upstream of nodA

We isolated and characterized nodA genes from photosynthetic and non-photosynthetic rhizobia nodulating the legume genus Aeschynomene, and found that the nodA sequence from photosynthetic stem-nodulating bacteria was phylogenetically distant from the other already described nodA genes. Characterization of the photosynthetic strain ORS285 common nod gene cluster (nodABC) showed, upstream of nodA, the presence of a new insertion sequence element belonging to the IS3 family and specific to a group of photosynthetic strains from Aeschynomene.

Effect of Bradyrhizobium photosynthesis on stem nodulation of Aeschynomene sensitiva

Some leguminous species of the genus Aeschynomene are specifically stem-nodulated by photosynthetic bradyrhizobia. To study the effect of bacterial photosynthesis during symbiosis, we generated a photosynthesis-negative mutant of the Bradyrhizobium sp. strain ORS278 symbiont of Aeschynomene sensitiva. The presence of a functional photosynthetic unit in bacteroids and the high expression of the photosynthetic genes observed in stem nodules demonstrate that the bacteria are photosynthetically active during stem symbiosis. Stem inoculation by the photosynthetic mutant gave a 50% decrease in stem-nodule number, which reduced nitrogen fixation activity and plant growth in the same proportion. These results indicate an important role of bacterial photosynthesis in the efficiency of stem nodulation.

Photosynthetic bradyrhizobia are natural endophytes of the African wild rice Oryza breviligulata

We investigated the presence of endophytic rhizobia within the roots of the wetland wild rice Oryza breviligulata, which is the ancestor of the African cultivated rice Oryza glaberrima. This primitive rice species grows in the same wetland sites as Aeschynomene sensitiva, an aquatic stem-nodulated legume associated with photosynthetic strains of Bradyrhizobium. Twenty endophytic and aquatic isolates were obtained at three different sites in West Africa (Senegal and Guinea) from nodal roots of O. breviligulata and surrounding water by using A. sensitiva as a trap legume. Most endophytic and aquatic isolates were photosynthetic and belonged to the same phylogenetic Bradyrhizobium/Blastobacter subgroup as the typical photosynthetic Bradyrhizobium strains previously isolated from Aeschynomene stem nodules. Nitrogen-fixing activity, measured by acetylene reduction, was detected in rice plants inoculated with endophytic isolates. A 20% increase in the shoot growth and grain yield of O. breviligulata grown in a greenhouse was also observed upon inoculation with one endophytic strain and one Aeschynomene photosynthetic strain. The photosynthetic Bradyrhizobium sp. strain ORS278 extensively colonized the root surface, followed by intercellular, and rarely intracellular, bacterial invasion of the rice roots, which was determined with a lacZ-tagged mutant of ORS278. The discovery that photosynthetic Bradyrhizobium strains, which are usually known to induce nitrogen-fixing nodules on stems of the legume Aeschynomene, are also natural true endophytes of the primitive rice O. breviligulata could significantly enhance cultivated rice production.

Isolation and characterization of canthaxanthin biosynthesis genes from the photosynthetic bacterium Bradyrhizobium sp. strain ORS278

A carotenoid biosynthesis gene cluster involved in canthaxanthin production was isolated from the photosynthetic Bradyrhizobium sp. strain ORS278. This cluster includes five genes identified as crtE, crtY, crtI, crtB, and crtW that are organized in at least two operons. The functional assignment of each open reading frame was confirmed by complementation studies.

Multivariate data analysis using D-optimal designs, partial least squares, and response surface modeling: A directional approach for the analysis of farnesyltransferase inhibitors

We have investigated the combined use of partial least squares (PLS) and statistical design principles in principal property space (PP-space), derived from principal component analysis (PCA), to analyze farnesyltransferase inhibitors in order to identify "activity trends" (an approach we call a "directional" approach) and quantitative structure-activity relationships (QSAR) for a congeneric series of inhibitors: the benzo[f]perhydroisoindole (BPHI) series. Trends observed in the PCA showed that the descriptors used were relevant to describe our structural data set by clearly identifying two well-defined structural subclasses of inhibitors. D-Optimal design techniques allowed us to define a training set for PLS study in PP-space. Models were derived for each biological assay under evaluation: the in vitro Ki-Ras and cellular HCT116 tests. Each of these assay-based sets was subdivided once more into two subsets according to two structural classes in this BPHI series as revealed by the PCA model. The response surface modeling (RSM) methodology was used for each subset, and the corresponding RSM plots helped us identify "activity trends" exploited to guide further analogue design. For more precise activity predictions more refined PLS models on constrained PP-spaces were developed for each subset. This approach was validated with predicted sets and demonstrates that useful information can be extracted from just a few very informative and representative compounds. Finally, we also showed the potential use of such a strategy at an early stage of an optimization process to extract the first "activity trends" that might support decision making and guide medicinal chemists in the initial design of new analogues and/or lead followup libraries.

Evidence for active efflux as the primary mechanism of resistance to ciprofloxacin in Salmonella enterica serovar typhimurium

The occurrence of active efflux and cell wall modifications were studied in Salmonella enterica serovar Typhimurium mutants that were selected with enrofloxacin and whose phenotypes of resistance to fluoroquinolones could not be explained only by mutations in the genes coding for gyrase or topoisomerase IV. Mutant BN18/21 exhibited a decreased susceptibility to ciprofloxacin (MIC = 0.125 microg/ml) but did not have a mutation in the gyrA gene. Mutants BN18/41 and BN18/71 had the same substitution, Gly81Cys in GyrA, but exhibited different levels of resistance to ciprofloxacin (MICs = 2 and 8 microg/ml, respectively). None of the mutants had mutations in the parC gene. Evidence for active efflux was provided by a classical fluorimetric method, which revealed a three- to fourfold decrease in ciprofloxacin accumulation in the three mutants compared to that in the parent strain, which was annulled by addition of the efflux pump inhibitor carbonyl cyanide m-chlorophenylhydrazone. In mutant BN18/71, a second fluorimetric method also showed a 50% reduction in the level of accumulation of ethidium bromide, a known efflux pump substrate. Immunoblotting and enzyme-linked immunosorbent assay experiments with an anti-AcrA antibody revealed that the resistance phenotype was strongly correlated with the expression level of the AcrAB efflux pump and suggested that decreased susceptibility to ciprofloxacin due to active efflux probably related to overproduction of this pump could occur before that due to gyrA mutations. Alterations were also found in the outer membrane protein and lipopolysaccharide profiles of the mutants, and these alterations were possibly responsible for the decrease in the permeability of the outer membrane that was observed in the mutants and that could act synergistically with active efflux to decrease the level of ciprofloxacin accumulation.

Occurrence of natural dixenic associations between the symbiont Photorhabdus luminescens and bacteria related to Ochrobactrum spp. in tropical entomopathogenic Heterorhabditis spp. (Nematoda, Rhabditida)

Bacteria naturally associated with the symbiont Photorhabdus luminescens subsp. akhurstii were isolated from the entomopathogenic nematode Heterorhabditis indica. Bacterial isolates distinct from P. luminescens subsp. akhurstii were obtained from 33% of the samples. Fourteen bacterial isolates, from nematodes collected from three different Caribbean islands, were characterized by conventional phenotypic tests, restriction fragment length polymorphism and sequence analyses of PCR-amplified 16S rRNA genes (16S rDNAs). Isolates were grouped into three genotypes, each one being associated with one Caribbean island. Phenotypic characteristics and 16S rDNA analysis showed that the Photorhabdus-associated bacteria were closely related to Ochrobactrum anthropi for the group from Guadeloupe, and to Ochrobactrum intermedium for the two groups from the Dominican Republic and Puerto Rico. No pathogenicity of the Ochrobactrum spp. to the insects Galleria mellonella and Spodoptera littoralis (Lepidoptera) was detected. Since Ochrobactrum spp. are considered as human opportunist pathogens, the mass production of entomopathogenic nematodes for biological control requires strict vigilance.

Comparative studies of mutations in animal isolates and experimental in vitro- and in vivo-selected mutants of Salmonella spp. suggest a counterselection of highly fluoroquinolone-resistant strains in the field

The occurrence of mutations in the genes coding for gyrase (gyrA and gyrB) and topoisomerase IV (parE and parC) of Salmonella typhimurium experimental mutants selected in vitro and in vivo and of 138 nalidixic acid-resistant Salmonella field isolates was investigated. The sequencing of the quinolone resistance-determining region of these genes in highly fluoroquinolone-resistant mutants (MICs of 4 to 16 microg/ml) revealed the presence of gyrA mutations at codons corresponding to Gly-81 or Ser-83, some of which were associated with a mutation at Asp-87. No mutations were found in the gyrB, parC, and parE genes. An assay combining allele-specific PCR and restriction fragment length polymorphism was developed to rapidly screen mutations at codons 81, 83, and 87 of gyrA. The MICs of ciprofloxacin for the field isolates reached only 2 microg/ml, versus 16 microg/ml for some in vitro-selected mutants. The field isolates, like the mutants selected in vivo, had only a single gyrA mutation at codon 83 or 87. Single gyrA mutations were also found in highly resistant in vitro-selected mutants (MIC of ciprofloxacin, 8 microg/ml), which indicates that mechanisms other than the unique modification of the intracellular targets could participate in fluoroquinolone resistance in Salmonella spp. A comparison of experimental mutants selected in vitro, field strains, and mutants selected in vivo suggests that highly fluoroquinolone-resistant strains are counterselected in field conditions in the absence of selective pressure.

Photosynthetic bradyrhizobia from Aeschynomene spp. are specific to stem-nodulated species and form a separate 16S ribosomal DNA restriction fragment length polymorphism group

We obtained nine bacterial isolates from root or collar nodules of the non-stem-nodulated Aeschynomene species A. elaphroxylon, A. uniflora, or A. schimperi and 69 root or stem nodule isolates from the stem-nodulated Aeschynomene species A. afraspera, A. ciliata, A. indica, A. nilotica, A. sensitiva, and A. tambacoundensis from various places in Senegal. These isolates, together with 45 previous isolates from various Aeschynomene species, were studied for host-specific nodulation within the genus Aeschynomene, also revisiting cross-inoculation groups described previously by D. Alazard (Appl. Environ. Microbiol. 50:732-734, 1985). The whole collection of Aeschynomene nodule isolates was screened for synthesis of photosynthetic pigments by spectrometry, high-pressure liquid chromatography, and thin-layer chromatography analyses. The presence of puf genes in photosynthetic Aeschynomene isolates was evidenced both by Southern hybridization with a Rhodobacter capsulatus photosynthetic gene probe and by DNA amplification with primers defined from photosynthetic genes. In addition, amplified 16S ribosomal DNA restriction analysis was performed on 45 Aeschynomene isolates, including strain BTAi1, and 19 reference strains from Bradyrhizobium japonicum, Bradyrhizobium elkanii, and other Bradyrhizobium sp. strains of uncertain taxonomic positions. The 16S rRNA gene sequence of the photosynthetic strain ORS278 (LMG 12187) was determined and compared to sequences from databases. Our main conclusion is that photosynthetic Aeschynomene nodule isolates share the ability to nodulate particular stem-nodulated species and form a separate subbranch on the Bradyrhizobium rRNA lineage, distinct from B. japonicum and B. elkanii.

Molecular characterization of the alpha-amylase genes of Lactobacillus plantarum A6 and Lactobacillus amylovorus reveals an unusual 3' end structure with direct tandem repeats and suggests a common evolutionary origin

The alpha-amylase gene (amyA) of Lactobacillus plantarum A6 was isolated from the genome by polymerase chain reaction with degenerated oligonucleotides, synthesized according to the tryptic peptide amino acid sequences of the purified enzyme. Nucleic acid sequence analysis revealed one open reading frame of 2739 bp encoding a 913 amino acid protein. The amylase appears to be divided into two equal parts. The N-terminal part has the typical characteristics of the well-known alpha-amylase family (65% identity with the alpha-amylase of Bacillus subtilis and 97% identity with the partial sequence available for the alpha-amylase of Lactobacillus amylovorus). The C-terminal part displays a fairly unusual structure. It consists of four direct tandem repeated sequences of 104 amino acids sharing 100% similarity. The complete nucleotide sequence of the alpha-amylase gene of L. amylovorus was also determined. An open reading frame of 2862 bp encoding a 954 amino acid protein was identified. Perfect homology between the two amyA genes was observed in the N-terminal region. The C-terminal part of L. amylovorus alpha-amylase also included tandem repeat units but striking differences were observed: (i) the addition of one repeat unit; (ii) a shorter, 91 amino acid repetition unit. These structural homologies suggest that both genes have a common ancestor and may have evolved independently by duplication with subsequent recombination and mutation.

Age and gender-related incidence of chronic renal failure in a French urban area: a prospective epidemiologic study

OBJECTIVE

To determine the age- and gender-related incidence of chronic renal failure in a French urban area.

METHODS

Prospective study of adult patients newly identified as having established, chronic renal failure defined by serum creatinine (Scr) > or = 200 mumol/l, with the cooperation of all nephrology and dialysis units in the Ile de France district (10,660,000 inhabitants) during a 1-year period.

RESULTS

2775 patients (1780 males, 995 females) were referred with Scr > or = 200 mumol/l between July 1991 and June 1992, an overall incidence of 260/million population. 847 had advanced renal failure (Scr > or = 500 mumol/l) and 541 patients (19.5%) were > or = 75 years of age. The age-related incidence was 92, 264, 523 and 619/million population in the age groups 20-39, 40-59, 60-74 and > or = 75 years old, respectively. The annual incidence was twice as high in males than in females up to 75 years and three times as high in patients > or = 75 years (1124 vs 356/million population). Based on the proportion of patients reaching end-stage renal failure within one year of referral, the minimal estimation of the need for supportive therapy is 81/million/year.

CONCLUSIONS

This epidemiological study in a large French urban area indicates an incidence of 260 patients per million population annually referred to nephrology units for chronic renal failure defined by Scr > or = 200 mumol/l, with a marked preponderance of males and a dramatic increase of incidence with age in both genders.