A proposed architecture for the central domain of the bacterial enhancer-binding proteins based on secondary structure prediction and fold recognition

The expression of genes transcribed by the RNA polymerase with the alternative sigma factor <r54 (Ecr54) is absolutely dependent on activator proteins that bind to enhancer-like sites, located far upstream from the promoter. These unique prokaryotic proteins, known as enhancer-binding proteins (EBP), mediate open promoter complex formation in a reaction dependent on NTP hydrolysis. The best characterized proteins of this family of regulators are NtrC and Nif A, which activate genes required for ammonia assimilation and nitrogen fixation, respectively. In a recent IRBM course (“Frontiers of protein structure prediction,” IRBM, Pomezia, Italy, 1995; see web site http://www.mrc-cpe.cam.uk/ irbm-course95/), one of us (J.O.) participated in the elaboration of the proposal that the Central domain of the EBPs might adopt the classical mononucleotide-binding fold. This suggestion was based on the results of a new protein fold recognition algorithm (Map) and in the mapping of correlated mutations calculated for the sequence family on the same mononucleotide-binding fold topology. In this work, we present new data that support the previous conclusion. The results from a number of different secondary structure prediction programs suggest that the Central domain could adopt an alfi topology. The fold recognition programs ProFIT 0.9, 3D PROFILE combined with secondary structure prediction, and 123D suggest a mononucleotide-binding fold topology for the Central domain amino acid sequence. Finally, and most importantly, three of five reported residue alterations that impair the Central domain ATPase activity of the Eo-54 activators are mapped to polypeptide regions that might be playing equivalent roles as those involved in nucleotide-binding in the mononucleotide-binding proteins. Furthermore, the known residue substitutions that alter the function of the Ecr54 activators, leaving intact the Central domain ATPase activity, are mapped on a region proposed to play an equivalent role as the effector region of the GTPase superfamily.     

Proferrorosamines and phytopathogenicity in Erwinia spp.

Proferrorosamine A (pFR A) of the plant pathogenic bacterium Erwinia rhapontici was shown to inhibit growth of wheat and cress seedlings at the 10 ppm level. When the seeds were continuously exposed to 100 ppm pFR A, the germination of cress and wheat seeds was inhibited up to 90% and 80%, respectively. The inhibition could be reversed through addition of equimolar amounts of ferrous iron, which indicates that the strong iron chelating capability of pFR A is responsible for the observed effect. The Fe(II) in the corresponding iron complex, ferrorosamine A, was found to be remarkably resistant towards oxidation by hydrogen peroxide and therefore redox-cycling in the Haber-Weiss cycle. It is thus conceivable that pFR A may also attenuate the generation of reactive hydroxyl radicals during the resistant and wound reaction. The apparent correlation between proferrorosamine production and virulence in erwiniae was further corroborated through the analysis of Erwinia persicinus, a newly described species. Using electrospray ionization mass spectrometry, E. persicinus was shown to produce pFR A and pFR B, and preliminary evidence for the phytopathogenicity of E. persicinus was found in cress. Inhibition of wheat seedlings by E. persicinus could not be demonstrated, but this may be due to technical difficulties or different host specificities. Taken together, our results indicate that the phytopathogenicity of E. rhapontici and E. persicinus may, as least in part, be due to the release of proferrorosamines.     

Effects of microwave exposure and temperature on survival of mice infected with Streptococcus pneumoniae

Female CD-1 mice were injected with an LD₅₀ dose of Streptococcus pneumoniae and then exposed to 2.45 GHz (CW) microwave radiation at an incident power density of 10 mW/cm² (SAR = 6.8 W/kg), 4 h/d for 5 d at ambient temperatures of 19 °C, 22 °C, 25 °C, 28 °C, 31 °C, 34 °C, 37 °C and 40 °C. Four groups of 25 animals were exposed at each temperature with an equal number of animals concurrently sham-exposed. Survival was observed for a 10-d period after infection. Survival of the sham-exposed animals increased as ambient temperature increased from 19 °C–34 °C. At ambient temperatures at or above 37 °C the heat induced in the body exceeded the thermoregulatory capacity of the animals and deaths from hyperthermia occurred. Survival of the microwave-exposed animals was significantly greater than the shams (～20%) at each ambient temperature below 34 °C. Based on an analysis of the data it appears that the hyperthermia induced by microwave exposure may be more effective in increasing survival in infected mice than hyperthermia produced by conventional methods (ie, high ambient temperature). Microwave radiation may be beneficial to infected animals at low and moderate ambient temperatures, but it is detrimental when combined with high ambient temperatures.     

Purification and characterization of Xanthomonas campestris pv. glycines exopolysaccharide

The exopolysaccharides (EPS) of virulent and avirulent strains of Xanthomonas campestris pv. glycines, causal agent of bacterial pustule disease of soybean, and one strain of the soybean non-pathogen X. c. pv. campestris were isolated, purified, and their compositions compared. EPS produced by X. c. pv. glycines in a completely defined medium appears to be identical to the well-characterized EPS produced by X. c. pv. campestris (commonly referred to as xanthan gum). The EPS of all strains was composed of the carbohydrates glucose, mannose and glucuronic acid with acetyl and pyruvyl substituents present. Permethylation analyses indicated EPS preparations had identical hexose substitution patterns. Avirulent strains of X. c. pv. glycines produced as much or more acidic EPS as did virulent strains in vitro. None of the EPS preparations were active as elicitors of the soybean pterocarpanoid phytoalexin glyceollin as determined by a soybean cotyledon bioassay.     

Cross-pathogenicity between Formae Speciales of Fusarium oxysporum, the Pathogens of Cucumber and Melon

The population structure of Fusarium oxysporum f. sp. cucumerinum was studied using the vegetative compatibility grouping (VCG) approach. All 37 of the examined isolates from Israel were assigned to VCG 0180, the major VCG found in North America and the Mediterranean region. Approximately two‐thirds of the tested isolates were pathogenic to both cucumber and melon, but cumulatively they were more aggressive on cucumber, their major host, than on melon. Disease symptoms on melon plants were less destructive and often expressed as growth retardation. Melon cultivars differing in Fom genes for resistance to F. oxysporum f. sp. melonis were inoculated with three isolates of F. oxysporum f. sp. cucumerinum. Results showed that Fom genes do not confer resistance to F. oxysporum f. sp. cucumerinum, although different horticultural types may respond differently to this pathogen. The reciprocal inoculation of F. oxysporum f. sp. melonis on cucumber, using four physiological races, did not result in disease symptoms or growth retardation. It is concluded that cucumerinum and melonis should remain two distinct formae speciales.     

A Phytoalexin-Like Flavonol Involved in the Carnation (Dianthus caryophyllus)-Fusarium oxysporum f. sp. dianthi Pathosystem

The fungitoxic flavonol triglycoside, kaempferide 3‐O‐[2^G‐β‐d ‐glucopyranosyl]‐β‐rutinoside, is a constituent of the carnation cultivar ‘Novada’, known as one of the most resistant cultivar to Fusarium oxysporum f. sp. dianthi, causative agent of Fusarium wilt. Due to its constitutive presence within the carnation tissues, this antifungal flavonol should be considered as a phytoanticipin; its biosynthesis, however, is stimulated by the inoculation with F. oxysporum f. sp. dianthi, just as is the rule for a typical phytoalexin. The results seem to indicate that in carnation the concentration of some preformed antifungal flavonoids may be significantly increased by a fungal presence: owing to their fungitoxic properties, these molecules could cooperate, together with the unconstitutive and postinfectional anthranilic acid‐derivative phytoalexins, to the plant defensive response against Fusarium attacks.     

Epidemiology of Pseudomonas syringae Pathovars Associated with Decline of Plum Trees in the Southwest of Germany

Plum decline was associated with Pseudomonas syringae pathovars syringae and morsprunorum in Baden‐Württemberg. The trunks of affected plum trees (Prunus domestica) were girdled by bacterial cankers resulting in sudden death of infected trees. Copper compounds that were applied extensively during leaf fall and bud burst, were not effective. A minority of P. syringae strains isolated from cankers on plum trees were moderately resistant, while most strains were sensitive to cupric ions. Invasions through blossoms, leaves and wounds during the vegetation period were limited to the infection sites and plum trees coped effectively with both P. syringae pathovars eliminating them eventually. Infections after dormancy including very rare leaf scar infections did not induce cankers on the trunk. However, infections of dormant trees through frost injuries, (pruning) wounds or non‐injurious ingress by freezing and thawing were serious, because they led to cankers girdling the trunk. Control strategies to manage plum decline have to be adapted to the disease cycle. They should concentrate on the dormant period beginning with early frosts in autumn and ending with bud burst.     

火山喷发后植被演替的影响因子

火山喷发后的植被演替是一个复杂的动态过程,受到多种因子的共同制约。揭示这一过程及其影响因子对研究火山干扰下的植被演替规律、预测未来植被演替动态具有重要的指导意义。本文综述了该领域的主要研究工作,分析了影响火山喷发后植被演替的主要因子,并对其研究方法进行总结和展望。影响火山喷发后植被演替的因子主要包括生物因子和非生物因子。生物因子包括残遗种和种子传播,它们为火山喷发后的植被演替提供繁殖体和种源。非生物因子包括火山喷发事件、火山基质、土壤因子和气候因子,它们是重要的外部因子。火山喷发事件分别从火山干扰尺度、严重度和频度3个方面制约植被演替;火山基质类型及其异质性程度决定了植被演替的轨迹,对植被演替动态起主导作用;火山喷发导致土壤和气候环境恶劣,在火山喷发后的早期演替阶段,植被演替速率低,随着土壤和气候环境得到改善,植被演替速率逐渐加快。此外,将模型与"3S"技术相结合在大尺度上研究火山喷发后的植被演替是一个新的研究视角。     

Microbial control of soil organic matter mineralization responses to labile carbon in subarctic climate change treatments

Half the global soil carbon (C) is held in high‐latitude systems. Climate change will expose these to warming and a shift towards plant communities with more labile C input. Labile C can also increase the rate of loss of native soil organic matter (SOM); a phenomenon termed ‘priming’. We investigated how warming (+1.1 °C over ambient using open top chambers) and litter addition (90 g m^?2 yr^?1) treatments in the subarctic influenced the susceptibility of SOM mineralization to priming, and its microbial underpinnings. Labile C appeared to inhibit the mineralization of C from SOM by up to 60% within hours. In contrast, the mineralization of N from SOM was stimulated by up to 300%. These responses occurred rapidly and were unrelated to microbial successional dynamics, suggesting catabolic responses. Considered separately, the labile C inhibited C mineralization is compatible with previously reported findings termed ‘preferential substrate utilization’ or ‘negative apparent priming’, while the stimulated N mineralization responses echo recent reports of ‘real priming’ of SOM mineralization. However, C and N mineralization responses derived from the same SOM source must be interpreted together: This suggested that the microbial SOM‐use decreased in magnitude and shifted to components richer in N. This finding highlights that only considering SOM in terms of C may be simplistic, and will not capture all changes in SOM decomposition. The selective mining for N increased in climate change treatments with higher fungal dominance. In conclusion, labile C appeared to trigger catabolic responses of the resident microbial community that shifted the SOM mining to N‐rich components; an effect that increased with higher fungal dominance. Extrapolating from these findings, the predicted shrub expansion in the subarctic could result in an altered microbial use of SOM, selectively mining it for N‐rich components, and leading to a reduced total SOM‐use.