Cloning of histidine genes of Azospirillum brasilense: organization of the ABFH gene cluster and nucleotide sequence of the hisB gene

Summary A cluster of four Azospirillum brasilense histidine biosynthetic genes, hisA, hisB, hisF and hisH, was identified on a 4.5 kb DNA fragment and its organization studied by complementation analysis of Escherichia coli mutations and nucleotide sequence. The nucleotide sequence of a 1.3 kb fragment that complemented the E. coli hisB mutation was determined and an ORF of 624 nucleotides which can code for a protein of 207 amino acids was identified. A significant base sequence homology with the carboxyterminal moiety of the E. coli hisB gene (0.53) and the Saccharomyces cerevisiae HIS3 gene (0.44), coding for an imidazole glycerolphosphate dehydratase activity was found. The amino acid sequence and composition, the hydropathic profile and the predicted secondary structures of the yeast, E. coli and A. brasilense proteins were compared. The significance of the data presented is discussed.Abbreviations IGP imidazole glycerolphosphate - HP histidinolphosphate     

Mutation frequency decline following chemical mutagenesis of Salmonella typhimurium

The occurrence is reported of a mutation frequency decline process (MFD) following treatment of Salmonella typhimurium strain trpC₃ with two chemical mutagens which give rise predominantly to suppressor revertants. With the carcinogen 4-nitroquinoline-N-oxide (4NQO) the results are analogous to those obtained for UV-mutagenesis. In the case of methoxynamine, the process is due to specific excision of premutational lesions, since lethality is low and lethal lesions are non-excisable. Mutants are described which cannot perform MFD of lesions induced by one or both of the chemical mutagens, indicating that the loss of revertants is in each case due to a bacteial repair system rather than to spontaneous degradation of the induced lesion. The mutants, however, were isolated because of an altered response to UV mutagenesis, viz., their ability to express UV-induced mutants in the absence of amino acids to stimulate active post-irradiation protein synthesis. In all other respects tested, their response to UV is identical with that of the parent strain. The hypothesis is discussed that the total absence of UV-induced revertants of the strain S. typhimurium trpC₃ when active protein synthesis is inhibited is due to two processes, first, rapid MFD due to the specific excision of pyrimidine dimers (the predominant UV-lesion) and secondly, the slow excision of other premutational damage which may be other photoproducts or secondary distortions caused by close juxtaposition of several pyrimidine dimers.     

1H, 15N and 13C resonance assignment of imidazole glycerol phosphate (IGP) synthase protein HisF from Thermotoga maritima

HisF comprises one half of the heterodimeric protein complex imidazole glycerol phosphate (IGP) synthase responsible for the fifth step of histidine biosynthesis. Here we report backbone and side chain assignments necessary for characterization of protein dynamics involved in the allosteric mechanism of IGP synthase.     

Risk assessment of submicron PM-bound hexavalent chromium during wintertime

This study reports health risk assessment of PM₁-bound carcinogenic hexavalent chromium [Cr(VI)] from central part of Indo-Gangetic plain (IGP) (PM₁: particulate matter with aerodynamic diameter ≤1µm). Cr(VI) concentration has been estimated utilizing spectrophotometer with a modified novel method. Average ratio of Cr(VI)/Cr_T was 0.39 ± 0.07 (Cr_T: Total chromium) in the central IGP (Kanpur). Our study reports that mass fraction of Cr(VI) averaging at 0.39 is ～3 times higher than that assumed conventionally [Cr(VI)/Cr_T: 1/7]. Cancer risk assessment has been performed by assessing excess cancer risk (ECR) for the Cr(VI). ECR determined due to Cr(VI) was 57 and 14.3 (in one million) for adults and children, respectively. Our study suggests that risk due to Cr(VI) reported in previous studies were being underestimated by a factor of three. The Cr(VI)/Cr_T average ratio of 0.39 determined in this study was utilized to calculate risk assessment due to Cr(VI) from other locations in the IGP. Owing to large population of India (～125 million), the cancer risk due to Cr(VI) inhalation itself would become very significant. Thus, future research should focus on metal speciation of PM-bound samples from different locations to better constraint the toxicological risk assessment on a regional-to-global scale.     

Effects of enhanced productivity of resources shared by predators in a food‐web module: Comparing results of a field experiment to predictions of mathematical models of intra‐guild predation

We compared the response to resource enhancement of a simple empirical model of intra‐guild predation (IGP) to the predictions of published, simple mathematical models of asymmetric IGP (a generalist IG Predator that feeds both on a specialist IG Prey and a Resource that it shares with the IG Prey). The empirical model was a food‐web module created by pooling species abundances across many families in a speciose community of soil micro‐arthropods into three categories: IG Predator (large predatory mites), IG Prey (small predatory mites), and a shared Resource (fungivorous mites and springtails). By pooling abundances of species belonging to broadly defined functional groups, we tested the hypothesis that IGP is a dominant organizing principle in this community. Simple mathematical models of asymmetric IGP predict that increased input of nutrients and energy to the shared Resource will increase the equilibrium density of Resource and IG Predator, but will decrease that of IG Prey. In a field experiment, we observed how the three categories of the empirical model responded to two rates of addition of artificial detritus, which enhanced the food of fungivores, the Resource of the IGP module. By the experiment''s end, fungivore densities had increased ~1.5× (ratio of pooled fungivore densities in the higher‐input treatment to plots with no addition of detritus), and densities of IG Predators had increased ~4×. Contrary to the prediction of mathematical models, IG Prey had not decreased, but instead had increased ~1.5×. We discuss possible reasons for the failure of the empirical model to agree with IGP theory. We then explore analogies between the behavior of the empirical model and another mathematical model of trophic interactions as one way to gain insights into the trophic connections in this community. We also propose one way forward for reporting comparisons of simple empirical and mathematical models.     

Potential predation of the exotic Amblyseius swirskii on Euseius concordis (Acari: Phytoseiidae), a predatory mite commonly found in Brazil

Intra/interspecific predation by A. swirskii.

Both Benin and Netherlands populations of Amblyseius swirskii engaged in cannibalism, and in predation on Euseius concordis larvae. Provision of pollen decreased cannibalism considerably, but had a smaller effect on predation on E. concordis. Potential effect of the eventual introduction of A. swirskii for control of Bemisia tabaci in Brazil is discussed.     

Intraguild predation in biological control: consideration of multiple resource species

Theoretical models of intraguild predation (IGP) predict that IGP decreases the effectiveness of biological control while many empirical studies do not agree with this prediction. In this study, I discuss the importance of explicit consideration of multiple resource species that has been neglected in most theoretical IGP models. In the previous models of IGP, a single resource species represented the pest species. However, there are multiple resource species (e.g., multiple pest species or aggregates of pest and non-pest species) in real systems. This study shows that models with multiple resource species can predict a variety of outcomes including those consistent with the empirical observations. The explicit consideration of resource species is useful for the future development of theories in biological control. Handling Editor: Helen Roy     

Intraguild predation and sublethal interactions between two zoophytophagous mirids,Macrolophus pygmaeus and Nesidiocoris tenuis

The omnivorous predators Macrolophus pygmaeus and Nesidiocoris tenuis (Hemiptera: Miridae) are important biological control agents of pests on tomato crops. In this study, potential intraguild predation (IGP) interactions between the two species were investigated on tomato. We examined: (a) the within plant distribution of both species in the field, (b) the within plant distribution of each predatory species when co-occurred at high densities on tomato caged plants, (c) their behavioral interactions when enclosed in experimental arenas and (d) the development young and old nymphs of M. pygmaeus when enclosed together with N. tenuis adults. Results revealed that the two predators showed a different distribution pattern on the plants, with N. tenuis exploiting mostly the upper part, whereas M. pygmaeus were mostly observed on the 5th to the 7th leaf from the top. However, when the predators co-occurred, N. tenuis or M. pygmaeus individuals were recorded with increased numbers on the lower or the higher part of the plant, respectively. In the presence of N. tenuis adult young nymphs of M. pygmaeus completed their development to the adult stage, when alternative prey (lepidopteran eggs) was present on the plant, however failed to reach adulthood in the absence of alternative prey. A high percentage of the dead nymphs found with their body fluids totally sucked indicating predation by N. tenuis. However, large 4th instar nymphs of M. pygmaeus were much less vulnerable to N. tenuis than younger. The behavior of N. tenuis was affected by the presence of M. pygmaeus, but at a rate similar to that when two individuals of N. tenuis were enclosed together. Contacts between the predators were recorded in a similar frequency in mono- and heterospecific treatments, whereas aggressive behavior was not observed. This study shows that intraguild interactions between M. pygmaeus and N. tenuis occur but are not intensive. The potential implications of the outcomes for biological control are discussed.     

Internal promoter of the tryptophan operon of Escherichia coli is located in a structural gene

The constitutive low-efficiency promoter site (P₂) near the middle of the tryptophan operon of Escherichia coli has been mapped by analysis of short deletions internal to the trp operon. Comparison of deletions which remove this internal promoter with those which retain it show that P₂ is located within trpD, the region coding for phosphoribosyl anthranilate transferase. P₂ maps near the operator-distal end of trpD, on the operator-proximal side of two trpD point mutants. Comparisons of strains with and without the P₂ site indicate that initiations at this promoter are responsible for synthesis of 80% of the trpC, trpB and trp A polypeptides present in repressed cells.