Cyclic Lipopeptide Surfactant Production by Pseudomonas fluorescens SS101 Is Not Required for Suppression of Complex Pythium spp. Populations

ABSTRACT Previously, the zoosporicidal activity and control of Pythium root rot of flower bulbs by Pseudomonas fluorescens SS101 was attributed, in part, to the production of the cyclic lipopeptide surfactant massetolide A. The capacity of strain SS101 and its surfactant-deficient massA mutant 10.24 to suppress populations and root infection by complex Pythium spp. communities resident in orchard soils was assessed on apple and wheat seedlings and on apple rootstocks. Both strains initially became established in soil and persisted in the rhizosphere at similar population densities; however, massA mutant 10.24 typically was detected at higher populations in the wheat rhizosphere and soil at the end of each experiment. Both strains effectively suppressed resident Pythium populations to an equivalent level in the presence or absence of plant roots, and ultimately suppressed Pythium root infection to the same degree on all host plants. When split-root plant assays were employed, neither strain suppressed Pythium spp. infection of the component of the root system physically separated from the bacterium, suggesting that induced systemic resistance did not play a role in Pythium control. Strain SS101 only marginally suppressed in vitro growth of Pythium spp. and growth was not inhibited in the presence of mutant 10.24. When incorporated into the growth medium, the cyclic lipopeptide massetolide A significantly slowed the rate of hyphal expansion for all Pythium spp. examined. Differences in sensitivity were observed among species, with Pythium heterothallicum, P. rostratum, and P. ultimum var. ultimum exhibiting significantly greater tolerance. Pythium spp. populations indigenous to the two soils employed were composed primarily of P. irregulare, P. sylvaticum, and P. ultimum var. ultimum. These Pythium spp. either do not or rarely produce zoospores, which could account for the observation that both SS101 and mutant 10.24 were equally effective in disease control. Collectively, the results showed that (i) Pseudomonas fluorescens SS101 is very effective in controlling diverse Pythium populations on different crops grown in different soils and (ii) production of the cyclic lipopeptide massetolide A does not play a significant role in disease suppression. Other, as yet undefined mechanisms appear to play a significant role in the interaction between P. fluorescens SS101 and soilborne Pythium spp. communities.

Flavonoid-induced expression of a symbiosis-related gene in the cyanobacterium Nostoc punctiforme

The flavonoid naringin was found to induce the expression of hrmA, a gene with a symbiotic phenotype in the cyanobacterium Nostoc punctiforme. A comparative analysis of several flavonoids revealed the 7-O-neohesperidoside, 4'-OH, and C-2-C-3 double bond in naringin as structural determinants of its hrmA-inducing activity.

Developmental regulation and asymmetric expression of the gene encoding Cx43 gap junctions in the mouse limb bud

The Gja1 gene encoding the gap junction connexin 43 (Cx43) is dynamically regulated during limb morphogenesis. Transcript expression is found in many regions of the limb bud known to be important in regulating limb growth and patterning. In the newly emerged limb bud, Gja1 transcripts are first expressed in the ventrodistal margin of the ectoderm, and later transcript expression is localized to the apical ectodermal ridge (AER). Interestingly, transcript expression in the ventrodistal ectoderm is initiated left/right asymmetrically, with some strain backgrounds showing reverse sidedness in the fore vs. hindlimb buds. In legless, a mouse mutant exhibiting both limb and left/right patterning defects, Gja1 transcripts could not be detected in this region. However, in the i.v./i.v. embryo, a mutant with randomization of body situs the same pattern of Gja1 asymmetry was found in the limb ectoderm regardless of body situs. This suggests that Gja1 transcript expression is not directly linked to signaling pathways involved in specification of the left/right axis. In addition to transcript expression in the apical ectodermal ridge, Gja1 transcripts were also found at high levels in the ventral ectoderm. In the limb bud mesenchyme, Gja1 transcripts were distributed in a posterior distal gradient, coincident with tissue known to have polarizing activity. With limb outgrowth and the initiation of limb mesenchyme condensation. Gja1 transcripts were localized in the presumptive progress zone, and in the condensing mesenchyme. In more proximal regions of the limb where mesenchyme differentiation has been initiated, Gja1 transcripts were expressed only in the outer mesenchymal cells comprising the presumptive perichondrium. Further analysis of transgenic mice ectopically expressing Wnt-1 in the limb mesenchyme revealed alterations in the pattern of Gja1 transcript expression in conjunction with the perturbation of limb mesenchyme condensation and differentiation. Together, these findings indicate that Cx43 gap junctions may mediate cell-cell interactions important in cell signaling processes involved in limb growth and patterning.

Cx43 gap junction gene expression and gap junctional communication in mouse neural crest cells

Although gap junctions are not known to be important in mediating cell-cell interactions amongst migratory cells, our studies showed that the connexin 43 (Cx43) gap junction gene is widely expressed in mouse neural crest cell lineages. Using in situ hybridization analysis, Cx43 expression was detected in presumptive neural crest cells emerging from the neural folds of the early postimplantation embryo. Neural crest expression of the Cx43 gap junction gene was also indicated by the analysis of transgenic mice containing a lacZ reporter construct driven by the Cx43 promoter. In neural tube explant cultures of these transgenic mice, lacZ expression was observed in the emerging neural crest outgrowths. Whole mount X-gal staining of these transgenic embryos at various stages of development showed lacZ expression in neural crest cells distributed along the entire craniocaudal axis, with expression found in both cranial and trunk neural crest cells contributing to a wide range of embryonic tissues. This included presumptive cardiac neural crest cells localized in the heart. In light of the widespread expression of Cx43 in neural crest cell lineages, dye injection studies, were carried out to determine if neural crest cells are functionally coupled via gap junctions. Such studies revealed extensive dye coupling among presumptive neural crest cells, thus demonstrating that these migratory cells are indeed gap junctional communication competent. In total, these observations suggest that gap junctions may play a role in mouse neural crest development. This possibility is particularly intriguing given the recent finding that the Cx43 knockout mice die of defects associated with the outflow tract [Reaume et al., 1995], a region of the heart in which neural crest cells are required for normal development.

No turning, a mouse mutation causing left-right and axial patterning defects

Patterning along the left/right axes helps establish the orientation of visceral organ asymmetries, a process which is of fundamental importance to the viability of an organism. A linkage between left/right and axial patterning is indicated by the finding that a number of genes involved in left/right patterning also play a role in anteroposterior and dorsoventral patterning. We have recovered a spontaneous mouse mutation causing left/right patterning defects together with defects in anteroposterior and dorsoventral patterning. This mutation is recessive lethal and was named no turning (nt) because the mutant embryos fail to undergo embryonic turning. nt embryos exhibit cranial neural tube closure defects and malformed somites and are caudally truncated. Development of the heart arrests at the looped heart tube stage, with cardiovascular defects indicated by ballooning of the pericardial sac and the pooling of blood in various regions of the embryo. Interestingly, in nt embryos, the direction of heart looping was randomized. Nodal and lefty, two genes that are normally expressed only in the left lateral plate mesoderm, show expression in the right and left lateral plate mesoderm. Lefty, which is normally also expressed in the floorplate, is not found in the prospective floor plate of nt embryos. This suggests the possibility of notochordal defects. This was confirmed by histological analysis and the examination of sonic hedgehog, Brachyury, and HNF-3 beta gene expression. These studies showed that the notochord is present in the early nt embryo, but degenerates as development progresses. Overall, these findings support the hypothesis that the notochord plays an active role in left/right patterning. Our results suggest that nt may participate in this process by modulating the notochordal expression of HNF-3 beta.

Heart and neural tube defects in transgenic mice overexpressing the Cx43 gap junction gene

Transgenic mice were generated containing a cytomegaloviral promoter driven construct (CMV43) expressing the gap junction polylpeptide connexin 43. RNA and protein analysis confirmed that the transgene was being expressed. In situ hybridization analysis of embryo sections revealed that transgene expression was targeted to the dorsal neural tube and in subpopulations of neural crest cells. This expression pattern was identical to that seen in transgenic mice harboring other constructs driven by the cytomegaloviral promoter (Kothary, R., Barton, S. C., Franz, T., Norris, M. L., Hettle, S. and Surani, M. A. H. (1991) Mech. Develop. 35, 25–31; Koedood, M., Fitchel, A., Meier, P. and Mitchell, P. (1995) J. Virol. 69, 2194–2207), and corresponded to a subset of the endogenous Cx43 expression domains. Significantly, dye injection studies showed that transgene expression resulted in an increase in gap junctional communication. Though viable and fertile, these transgenic mice exhibited reduced postnatal viability. Examination of embryos at various stages of development revealed developmental perturbations consisting of cranial neural tube defects (NTD) and heart malformations. Interestingly, breeding of the CMV43 transgene into the Cx43 knockout mice extended postnatal viability of mice homozygote for the Cx43 knockout allele, indicating that the CMV43 trangsene may partially complement the Cx43 deletion. Both the Cx43 knockout and the CMV43 transgenic mice exhibit heart defects associated with malformations in the conotruncus, a region of the heart in which neural crest derivatives are known to have important roles during development. Together with our results indicating neural-crest-specific expression of the transgene in our CMV-based constructs, these observations strongly suggest a role for Cx43-mediated gap junctional communication in neural crest development. Furthermore, these observations indicate that the precise level of Cx43 function may be of critical importance in downstream events involving these migratory cell populations. As such, the CMV43 mouse may represent a powerful new model system for examining the role of extracardiac cell populations in cardiac morphogenesis and other developmental processes.

A polyketide-synthase-like gene is involved in the synthesis of heterocyst glycolipids in Nostoc punctiforme strain ATCC 29133

A Tn5-1063-derived mutant of Nostoc punctiforme strain ATCC 29133 was unable to fix N2 in air although it reduced acetylene in the absence of O2. Mutant strain UCD 307 formed cells morphologically similar to heterocysts, but it failed to synthesize the characteristic heterocyst glycolipids. Sequence analysis around the site of insertion revealed an ORF of 3,159 base pairs, termed hglE. hglE putatively encodes a 115.4-kDa protein containing two domains with conserved amino acid sequences identified with acyl transferase and the chain length factor variation of beta-ketoacyl synthase active sites. These active sites are characteristic of polyketide and fatty acid synthases. The N. punctiforme strain 29133 hglE gene is transcribed only under nitrogen-limiting growth conditions. The hglE gene, or similar sequences, was found in all other heterocyst-forming cyanobacteria surveyed and was absent in unicellular Synechococcus sp. strain PCC 7942. Based on these results, we propose that the synthesis of heterocyst glycolipids follows a pathway characteristic of polyketide synthesis and involves similar large, multienzyme complexes.

A hormogonium regulating locus, hrmUA, of the cyanobacterium Nostoc punctiforme strain ATCC 29133 and its response to an extract of a symbiotic plant partner Anthoceros punctatus

Transposon-generated mutant strain UCD 328 of Nostoc punctiforme strain ATCC 29133 has a phenotype of an increased sensitivity to a hormogonium-inducing factor exuded by a symbiotic plant partner, Anthoceros punctatus, and an initial increased hormogonium-dependent infection of the plant. Sequence analysis showed that the transposition site in strain UCD 328 lies within a 1,251-bp open reading frame (ORF), designated hrmA, that displays no significant similarity to known database sequences. A second, 837-bp ORF (hrmU) ends 2 bp 5' from the start of hrmA and has the signature sequences belonging to a family of NAD(P)H-dependent oxidoreductases. Strains having insertional mutations in hrmU or hrmA reproduce the strain UCD 328 phenotype. Transcriptional fusions of luxAB to hrmU or hrmA show an 8- to 10-fold peak increase in luciferase activity 13 to 20 h after the start of incubation in the presence of an aqueous extract of A. punctatus. A promoter induced by the extract was deduced to be between 2.0 to 3.4 kb from the translational start of hrmU. A multicopy plasmid that contains hrmUA within a 6.2-kb fragment conferred an increased infection phenotype on wild-type N. punctiforme 29133. This plasmid and another plasmid containing 4.4 kb of DNA 5' of the transposition site prevented extract-dependent induction of hrmA-luxAB transcription in strain UCD 328, implicating titration of some trans-activator(s) by the cloned fragments. We hypothesize a role for hrmUA in the inhibition of hormogonium formation by the metabolism of an unknown hormogonium-regulating metabolite.

The devR gene product is characteristic of receivers of two-component regulatory systems and is essential for heterocyst development in the filamentous cyanobacterium Nostoc sp. strain ATCC 29133

Strain UCD 311 is a transposon-induced mutant of Nostoc sp. strain ATC C 29133 that is unable to fix nitrogen in air but does so under anoxic conditions and is able to establish a functional symbiotic association with the hornwort Anthoceros punctatus. These properties of strain UCD 311 are consistent with previous observations that protection against oxygen inactivation of nitrogenase is physiologically provided within A. punctatus tissue. Upon deprivation of combined nitrogen, strain UCD 311 clearly differentiates heterocysts and contains typical heterocyst-specific glycolipids; it also makes apparently normal akinetes upon phosphate starvation. Sequence analysis adjacent to the point of the transposon insertion revealed an open reading frame designated devR. Southern analysis established that similar sequences are present in other heterocyst-forming cyanobacteria. devR putatively encodes a protein of 135 amino acids with high similarity to the receiver domains of response regulator proteins characteristics of two-component regulatory systems. On the basis of its size and the absence of other functional domains, DevR is most similar to CheY and Spo0F. Reconstruction of the mutation with an interposon vector confirmed that the transposition event was responsible for the mutant phenotype. The presence of wild-type devR on a plasmid in strain UCD 311 restored the ability to fix nitrogen in air. While devR was not essential for differentiation of akinetes, its presence in trans in Nostoc sp. strain ATCC 29133 stimulated their formation to above normal levels in aging medium. On the basis of RNA analysis, devR is constitutively expressed with respect to the nitrogen source for growth. The devR gene product is essential to the development of mature heterocysts and may be involved in a sensory pathway that is not directly responsive to cellular nitrogen status.

Transposon mutagenesis of Nostoc sp. strain ATCC 29133, a filamentous cyanobacterium with multiple cellular differentiation alternatives

Nostoc sp. strain ATCC 29133 (PCC 73102; Nostoc 29133) is a symbiotically-competent, facultatively heterotrophic, diazotrophic cyanobacterium with the capacity to differentiate specialized cells such as heterocysts, akinetes and hormogonial filaments. We have optimized several methods for physiological and molecular genetic analysis of Nostoc 29133. By use of a Tn5 derivative, Tn5-1063 (Km(r)Bm(r)Sm(r)), delivered by conjugation from Escherichia coli, antibiotic-resistant mutants of Nostoc 29133 were generated at a frequency of approximately 1 x 10(-6), 0.4% of which expressed a nitrogen fixation (heterocyst) defective phenotype. Mutant strain UCD 328 was isolated after co-culture of 86 Nostoc 29133::Tn5-1063 clones with the symbiotic plant partner, Anthoceros punctatus; strain UCD 328 expressed a symbiotic phenotype of increased frequency of hormogonia-dependent infection. The transposon and flanking genomic DNA was recovered from strain UCD 328, the mutation and phenotype reconstructed by homologous recombination in Nostoc 29133, and the transposition site identified from a Nostoc 29133 genomic library. Transposon mutagenesis has thus provided the means for isolation and identification of developmental and symbiotic-specific genes of Nostoc 29133.

Direct regulation of Na(+)-dependent myo-inositol transport by sugars in retinal pigment epithelium: role of phorbol ester and staurosporin

An Na(+)-dependent active process for myo-inositol (MI) uptake, sharing a common carrier system with glucose and sensitive to phlorizin, was previously established in primary cultures of bovine retinal pigment epithelial (RPE) cells (26, 32). The present report further examines the nature of glucose-induced inhibition of MI transport in primary cultures of RPE cells. RPE cells were grown in supplemented Dulbecco's modification of Eagle's medium (DMEM) containing 5 mM D-glucose (basic growth media) or 40 mM D-glucose or its nonmetabolizable analogue, alpha-methyl-D-glucoside (alpha MG); 1-5 mM nonradioactive MI, pyruvate, or lactate; or 0.2-20 microM phorbol 12-myristate 13-acetate (TPA) or straurosporin (modified growth media), for up to 4 weeks. The capacity of RPE cells to accumulate 3H-MI (ratios of intracellular transported radioactive MI, [MI]i, to external free MI concentration, [MI]i/[MI]o) decreased by up to 41% or 34% when cells were grown for 10 days or longer with 40 mM D-glucose or 40 mM alpha MG, respectively, compared to cells grown in basic growth media. The rate of uptake of 3H-MI also was reduced to 63 +/- 15% or 48 +/- 8% of the control values when cells were fed 1 or 5 mM nonradioactive MI, respectively. In addition, cellular capacity to bind to [3H]phlorizin was reduced to 52 +/- 7%, 61 +/- 5%, or 38 +/- 6% of the controls when RPE cells were fed 40 mM D-glucose, 40 mM alpha MG, or 5 mM nonradioactive MI, respectively. Growth media containing either pyruvate or lactate, the glucose metabolites, did not suppress the ability of RPE cells to accumulate MI. An 18 +/- 8% reduction in [3H]thymidine incorporation into DNA occurred when cells were grown in 40 mM glucose for 12-14 days, compared to cells grown with 5 mM glucose. Chronic treatment (12-14 days) of the cells with phorbol ester, an activator of protein kinase C, caused up to twofold increase in MI uptake, [3H]phlorizin binding, cell number, and DNA synthesis. However, when the rates of MI uptake into cells grown in basic growth media or TPA-treated media were normalized to cell number, no significant difference in MI uptake was found between the treated and untreated cells. Addition of staurosporin, a protein kinase C inhibitor, together with TPA, in the growth media reversed the phorbol-induced increase of MI uptake.(ABSTRACT TRUNCATED AT 400 WORDS)

The cytoskeletal system of mammalian primitive erythrocytes: studies in developing marsupials

Seeking to resolve conflicting literature on cytoskeletal structure in mammalian "primitive" generation erythrocytes, we have utilized the circulating blood of developing marsupials. In young of the Tammar Wallaby (Macropus eugenii) and the Gray Short-tailed Opossum (Monodelphis domestica), relatively large, nucleated primitive erythrocytes constituted nearly 100% of the circulating population at birth (= day 0) and in fetuses (Tammar) several days before birth. These cells were discoidal or elliptical, and flattened except for a nuclear bulge. Their cytoskeletal system, consisting of a marginal band of microtubules enclosed within a cell surface-associated network (membrane skeleton), closely resembled that of non-mammalian vertebrate erythrocytes. By day 2 or 3, much smaller anucleate erythrocytes of "definitive" morphology, lacking marginal bands, appeared in abundance. These accounted for greater than 90% of the circulating population of both species by day 6-8. Non-nucleated erythrocytes of a different type, constituting 1-6% of the cells in most blood samples up to day 7, were identified as anucleate primitives on the basis of size, shape, and presence of a marginal band. Thus, loss of erythrocyte nuclei in mammals appears to begin earlier than generally recognized, i.e., in the primitive generation. Counts of these anucleate primitives in young of various ages implicated nucleated primitives as their probable source. Pointed erythrocytes, occasionally found in younger neonates of both species, occurred in greatest number in fetuses (Tammar) prior to birth. This is in accord with previous work on non-mammalian vertebrates suggesting that such cells are morphogenetic intermediates. The results confirm the long-suspected similarity between mammalian primitive erythrocytes and the nucleated erythrocytes of all non-mammalian vertebrates.

The effects of growth factors and serum on DNA synthesis and differentiation in thyroid cells in culture

The effects of three putative growth factors and serum on [Me-3H]thymidine and Na125I incorporation into thyroid cell cultures have been examined. We found that serum and EGF could stimulate radioactively labelled thymidine incorporation into confluent cultures. However, both factors completely inhibited iodine uptake and organification at low concentrations. Insulin also stimulated [Me-3H]thymidine incorporation but had no adverse effect on thyroid differentiated function. TSH examined under the same conditions was not a growth factor but was essential to maintain differentiated functions. We conclude that thyroid growth and differentiation are not mutually exclusive processes. However, EGF and serum inhibit thyroid differentiated function at very low concentrations. Elucidation of the physiological role of these factors and their mechanism of action may lead to a greater understanding of thyroid hormone biosynthesis.

Detection threshold microstructure and its effect on temporal integration data

Auditory detection thresholds were measured at several preselected frequencies using an adaptive 2IFC procedure. Results confirm the existence of shifts in detection threshold ranging from 2 to 14 dB with quite small changes in signal frequency. There does not appear to be a uniform pattern associated with the microstructure of the detection threshold curve. An additional experiment was performed to determine the effect of signal duration on detection threshold microstructure. Results indicate that the temporal integration function is considerably steeper for more sensitive frequencies (3.7 dB/doubling of duration), than for less sensitive frequencies (1.7 dB/doubling). This probably is not due to differences in processing as much as it is to the effect of the energy spread associated with decreasing signal duration.