How the timberline formed: altitudinal changes in stand structure and dynamics around the timberline in central Japan

Background and Aims

Altitudinal timberlines are thought to move upward by global warming, a crucial topic in ecology. Tall tree species (the conifer Abies mariesii and the deciduous broad-leaved Betula ermanii) dominate the sub-alpine zone between 1600 and 2500 m a.s.l., the timberline, on Mount Norikura in central Japan. Dwarf pine Pinus pumila dominates above the timberline to near the summit (3026 m a.s.l.). This study evaluated how the timberline formed on Mount Norikura by examining altitudinal changes in stand structure and dynamics around the timberline.

Methods

One hundred and twenty-five plots of 10 m × 10 m were established around the timberline (2350–2600 m a.s.l.). Trunk diameter growth rate during 6 years was examined for A. mariesii, B. ermanii and P. pumila. Mortality during this period and mechanical damage scars on the trunks and branches due to strong wind and snow were examined for A. mariesii only.

Key Results

The density, maximum trunk height and diameter of A. mariesii in plots decreased with altitude. The maximum trunk height of B. ermanii decreased with altitude, but density and maximum trunk diameter did not decrease. In contrast, the density of P. pumila abruptly increased from around the timberline. A strong negative correlation was found between the densities of P. pumila and tall tree species, indicating their interspecific competition. Trunk diameter growth rates of A. mariesii and B. ermanii did not decrease with altitude, suggesting that these two tall tree species can grow at the timberline. The ratio of trees with mechanical damage scars increased with altitude for A. mariesii, a tendency more conspicuous for larger trees. The mortality of larger A. mariesii was also greater at higher altitude. Tall tree species may not increase their trunk height and survive around the timberline because of mechanical damage.

Conclusions

This study suggests that the altitudinal location of the timberline is mainly affected by mechanical damage due to strong wind and snow rather than by growth limitation due to low temperature. Therefore, the timberline would not move upward even under global warming if these growth and mortality characteristics do not change for a long time.     

Using the biomass-ratio and idiosyncratic hypotheses to predict mixed-species litter decomposition

Background and Aims

Microsporogenesis leading to monosulcate pollen grains has already been described for a wide range of monocot species. However, a detailed study of additional callose deposition after the completion of the cleavage walls has been neglected so far. The study of additional callose deposition in monosulcate pollen grain has gained importance since a correlation between additional callose deposition and aperture location has recently been revealed.

Methods

Microsporogenesis is described for 30 species belonging to eight families of the monocots: Acoraceae, Amaryllidaceae, Alstroemeriaceae, Asparagaceae, Butomaceae, Commelinaceae, Liliaceae and Xanthorrhoeaceae.

Key Results

Five different microsporogenesis pathways are associated with monosulcate pollen grain. They differ in the type of cytokinesis, tetrad shape, and the presence and shape of additional callose deposition. Four of them present additional callose deposition.

Conclusions

In all these different microsporogenesis pathways, aperture location seems to be linked to the last point of callose deposition.     

Window of opportunity: an episode of recruitment in a Banksia hybrid zone demonstrates continuing hybridization and phenotypic plasticity

Background and Aims

In perennial plants (especially post-fire resprouters), extant populations may reflect recruitment events in the distant past. This is true of hybrid zones formed by two Banksia species of swamps and woodlands in south-eastern Australia, Banksia robur and B. oblongifolia. Both resprout after fire but recruitment is dependent on periodic fires. Although plants of intermediate morphology have also been identified as hybrids using allozyme markers, the extent of ongoing hybridization is unknown. This study investigates whether both microsatellite markers and morphological measurements can be used to distinguish between the two species and their hybrids. A recent recruitment event and microsatellite markers allow the frequency of ongoing hybridization to be estimated, and also the effects of environmental variation on the morphology of plants and seedlings to be tested.

Methods

Variation at seven microsatellite loci was scored and seven leaf characteristics within putatively pure stands and mixed stands of both species were measured, revealing that the two species were genetically and morphologically distinct and that mixed stands also contained genetically and sometimes morphologically distinct hybrids. An opportunity created by wildfires was used to analyse the genetics and morphometrics of adults and seedlings from two hybrid zones.

Key Results

Approximately 9 % of adults and 21 % of seedlings were identified as genetic hybrids in both hybrid zones. Within these sites, the genotype of mature plants correlated well with morphology, except for some hybrid plants that had parental morphology. However, seedling morphology was highly variable and insufficient to describe the composition of the hybrid zone in this cohort. Greater phenotypic plasticity was evident among seedlings growing within the hybrid zones than seedlings growing in pots.

Conclusions

The hybrid zones are complex and the range of genotypes detected in seedlings reveals both continuing hybridization and introgression.     

Larval competition alters susceptibility of adult Aedes mosquitoes to dengue infection

Dengue, the most important human arboviral disease, is transmitted primarily by Aedes aegypti and, to a lesser extent, by Aedes albopictus. The current distributions of these invasive species overlap and are affected by interspecific larval competition in their container habitats. Here we report that competition also enhances dengue infection and dissemination rates in one of these two vector species. We determined the effects of competition on adult A. aegypti and A. albopictus, comparing their susceptibility to infection with a Southeast Asian strain of dengue-2 virus. High levels of intra- or interspecific competition among larvae enhanced the susceptibility of A. albopictus to dengue virus infection and potential for transmission, as indicated by disseminated infections. Doubling the number of competing larvae (A. albopictus or A. aegypti), led to a significant (more than 60%) increase in the proportion of A. albopictus with disseminated dengue-2 infection. Competition-enhanced vector competence appears to result from a reduction in 'barriers' (morphological or physiological) to virus infection and dissemination and may contribute to the importance of A. albopictus in dengue transmission. Similar results for other unrelated arboviruses suggest that larval competition, common in mosquitoes, should be considered in estimates of vector competence for pathogens that infect humans.     

Genetic diversity and population structure in the tomato-like nightshades Solanum lycopersicoides and S. sitiens

Background and Aims

Two closely related, wild tomato-like nightshade species, Solanum lycopersicoides and Solanum sitiens, inhabit a small area within the Atacama Desert region of Peru and Chile. Each species possesses unique traits, including abiotic and biotic stress tolerances, and can be hybridized with cultivated tomato. Conservation and utilization of these tomato relatives would benefit from an understanding of genetic diversity and relationships within and between populations.

Methods

Levels of genetic diversity and population genetic structure were investigated by genotyping representative accessions of each species with a set of simple sequence repeat (SSR) and allozyme markers.

Key Results

As expected for self-incompatible species, populations of S. lycopersicoides and S. sitiens were relatively diverse, but contained less diversity than the wild tomato Solanum chilense, a related allogamous species native to this region. Populations of S. lycopersicoides were slightly more diverse than populations of S. sitiens according to SSRs, but the opposite trend was found with allozymes. A higher coefficient of inbreeding was noted in S. sitiens. A pattern of isolation by distance was evident in both species, consistent with the highly fragmented nature of the populations in situ. The populations of each taxon showed strong geographical structure, with evidence for three major groups, corresponding to the northern, central and southern elements of their respective distributions.

Conclusions

This information should be useful for optimizing regeneration strategies, for sampling of the populations for genes of interest, and for guiding future in situ conservation efforts.     

Tree litter and forest understorey vegetation: a conceptual framework to understand the effects of tree litter on a perennial geophyte, Anemone nemorosa

Background and Aims

Litter is a key factor in structuring plant populations, through positive or negative interactions. The litter layer forms a mechanical barrier that is often strongly selective against individuals lacking hypocotyle plasticity. Litter composition also interacts with plant growth by providing beneficial nutrients or, inversely, by allowing harmful allelopathic leaching. As conspicuous litter fall accumulation is often observed under deciduous forests, interactions between tree litter and understorey plant populations are worthy of study.

Methods

In a 1-year ex-situ experiment, the effects of tree litter on the growth of Anemone nemorosa, a small perennial forest geophyte, were investigated. Three ‘litter quantity’ treatments were defined, representative of forest floor litter (199, 356·5 and 514 g m⁻²), which were crossed with five ‘litter composition’ treatments (Quercus petraea, Fagus sylvatica, Carpinus betulus, Q. petraea + F. sylvatica and Q. petraea + C. betulus), plus a no-litter control. Path analysis was then used to investigate the pathways linking litter characteristics and components of adult plant growth.

Key Results

As expected, the heavier the litter, the longer the petiole; rhizome growth, however, was not depreciated by the litter-induced petiole lengthening. Both rhizome mass increment and number of initiated buds marginally increased with the amount of litter. Rhizome mass increment was in fact determined primarily by leaf area and leaf life span, neither of which was unequivocally correlated with any litter characteristics. However, the presence of litter significantly increased leafing success: following a late frost event, control rhizomes growing in the absence of litter experienced higher leaf mortality before leaf unfolding.

Conclusions

The study questions the role of litter as a physical or chemical barrier to ground vegetation; to better understand this role, there is a need for ex-situ, longer-term experiments coupled with in-situ observations in the forest.     

Spatial genetic structure of Aquilegia taxa endemic to the island of Sardinia

Background and Aims

The Mediterranean Basin is one of the most important regions for the Earth''s plant biodiversity; however, the scarcity of studies on fine scale patterns of genetic variation in this region is striking. Here, an assessment is made of the spatial genetic structure of all known locations of the three Sardinian endemic species of Aquilegia in order to determine the relative roles of gene flow and genetic drift as underlying evolutionary forces canalizing the divergence of Sardinian Aquilegia taxa, and to see if the spatial genetic structure found fits the current taxonomic differentiation of these taxa.

Methods

DNA from 89 individuals from all known locations of Aquilegia across Sardinia was analysed by means of amplified fragment length polymorphism (AFLP) markers. Both principal co-ordinates analysis (PCoA) and Bayesian clustering analyses were used to determine the spatial genetic structure irrespective of any taxonomic affiliation. Historical effects of gene flow and genetic drift were assessed by checking for the existence of isolation-by-distance patterns.

Key Results

STRUCTURE and PCoA analyses revealed a pattern of genetic variation geographically structured into four spatial genetic groups. No migration–drift equilibrium was detected for Aquilegia in Sardinia, when analysed either as a whole or in individual groups. The scenario approached a Case III pattern sensu Hutchinson and Templeton, which is associated with extreme isolation conditions where genetic drift has historically played a dominant role over gene flow.

Conclusions

The pattern of genetic variation of Sardinian taxa of Aquilegia indicates that genetic drift has been historically more influential than gene flow on population structure of Sardinian species of Aquilegia. Limited seed dispersal and divergent selection imposed by habitat conditions have been probably the main causes reinforcing post-Pleistocene geographical isolation of Aquilegia populations. The spatial genetic structure found here is not fully compatible with current taxonomic affiliations of Sardinian Aquilegia taxa. This is probably a consequence of the uncoupling between morphological and genetic patterns of differentiation frequently found in recently radiated taxa.     

Evaluation of Several Approaches to Manage Meloidogyne incognita and Cotton Seedling Disease Complexes in the High Plains of Texas

Field experiments were conducted for control of the southern root-knot nematode (Meloidogyne incognita) and cotton seedling disease fungi (primarily Thielaviopsis basicola) in one naturally infested field during 1999 and 2000 and in three additional fields in 2000. Treatments included: seed-applied fungicides (triadimenol + mefenoxam + thiram and carboxin + PCNB + mefenoxam), cultivars (Paymaster [PM] 2326 RR and PM 2200 RR), and a nematicide (aldicarb at 0.83 kg a.i/ha). Plant stands were higher (P = 0.02) in the presence of aldicarb (77% emergence) than in its absence (74% emergence). Hypocotyl disease symptom ratings were lower (P = 0.0001) following triadimenol + mefenoxam + thiram seed treatment (0.53) as compared with carboxin + PCNB + mefenoxam (0.93). Root necrosis was lower (P = 0.002) following triadimenol + mefenoxam + thiram seed treatment (27%) as compared with carboxin + PCNB + mefenoxam (34%). In one field, in both years, aldicarb was associated with more root necrosis (58%) than in its absence (46%) (P = 0.004). At three other sites aldicarb did not affect root necrosis. Population densities of Meloidogyne incognita eggs and juveniles at midseason were greater (P = 0.005, P = 0.003, respectively) on PM 2200 RR (less resistant) than on PM 2326 RR (more resistant). Yield was affected by the plant genotype by aldicarb interaction (P = 0.02) but not by seed treatments. Aldicarb effect on yield was dependent on cultivar, whereas affect of seed treatment on root health was consistent and independent of cultivar and aldicarb. No conditions were identified when use of triadimenol + mefenoxam was detrimental.     

Monitoring long-term evolutionary changes following Wolbachia introduction into a novel host: the Wolbachia popcorn infection in Drosophila simulans

Wolbachia may act as a biological control agent for pest management; in particular, the Wolbachia variant wMelPop (popcorn) shortens host longevity and may be useful for dengue suppression. However, long-term changes in the host and Wolbachia genomes can alter Wolbachia spread and/or host effects that suppress disease. Here, we investigate the phenotypic effects of wMelPop in a non-native host, Drosophila simulans, following artificial transinfection approximately 200 generations ago. Long-term rearing and maintenance of the bacteria were at 19°C in the original I-102 genetic background that was transinfected with the popcorn strain. The bacteria were then introgressed into three massbred backgrounds, and tetracycline was used to create uninfected sublines. The effect of wMelPop on longevity in this species appears to have changed; longevity was no longer reduced at 25°C in some nuclear backgrounds, reflecting different geographical origin, selection or drift, although the reduction was still evident for flies held at 30°C. Wolbachia influenced productivity and viability, and development time in some host backgrounds. These findings suggest that long-term attenuation of Wolbachia effects may compromise the effectiveness of this bacterium in pest control. They also emphasize the importance of host nuclear background on Wolbachia phenotypic effects.     

Fatal attraction: adaptations to prey on native frogs imperil snakes after invasion of toxic toads

Adaptations that enhance fitness in one situation can become liabilities if circumstances change. In tropical Australia, native snake species are vulnerable to the invasion of toxic cane toads. Death adders (Acanthophis praelongus) are ambush foragers that (i) attract vertebrate prey by caudal luring and (ii) handle anuran prey by killing the frog then waiting until the frog''s chemical defences degrade before ingesting it. These tactics render death adders vulnerable to toxic cane toads (Bufo marinus), because toads elicit caudal luring more effectively than do native frogs, and are more readily attracted to the lure. Moreover, the strategy of delaying ingestion of a toad after the strike does not prevent fatal poisoning, because toad toxins (unlike those of native frogs) do not degrade shortly after the prey dies. In our laboratory and field trials, half of the death adders died after ingesting a toad, showing that the specialized predatory behaviours death adders use to capture and process prey render them vulnerable to this novel prey type. The toads'' strong response to caudal luring also renders them less fit than native anurans (which largely ignored the lure): all toads bitten by adders died. Together, these results illustrate the dissonance in behavioural adaptations that can arise following the arrival of invasive species, and reveal the strong selection that occurs when mutually naive species first interact.     

The shape of things to come: woodland herb niche contraction begins during recruitment in mesic forest microhabitat

Natural abundance is shaped by the abiotic requirements and biotic interactions that shape a species' niche, yet these influences are rarely decoupled. Moreover, most plant mortality occurs during early life stages, making seed recruitment critical in structuring plant populations. We find that natural abundance of two woodland herbs, Hexastylis arifolia and Hepatica nobilis, peaks at intermediate resource levels, a pattern probably formed by concurrent abiotic and biotic interactions. To determine how this abundance patterning reflects intrinsic physiological optima and extrinsic biotic interactions, we translocate adults and seeds to novel locations across experimentally extended abiotic gradients. These experiments indicate that the plant distributions probably reflect biotic interactions as much as physiological requirements, and that adult abundance provides a poor indication of the underlying niche requirements. The positive response exhibited by adult transplants in the wettest conditions is offset by increased fungal attack on buried seeds consistent with peak natural abundance where soil moisture is intermediate. This contraction of niche space is best described by Connell's model--species are limited by physiological tolerances where resources are low and biotic interactions where resources are high.     

Crystal structure of the arginine repressor protein in complex with the DNA operator from Mycobacterium tuberculosis

The arginine repressor (ArgR) from Mycobacterium tuberculosis (Mtb) is a gene product encoded by the open reading frame Rv1657. It regulates the l-arginine concentration in cells by interacting with ARG boxes in the promoter regions of the arginine biosynthesis and catabolism operons. Here we present a 2.5-Å structure of MtbArgR in complex with a 16-bp DNA operator in the absence of arginine. A biological trimer of the protein-DNA complex is formed via the crystallographic 3-fold symmetry axis. The N-terminal domain of MtbArgR has a winged helix-turn-helix motif that binds to the major groove of the DNA. This structure shows that, in the absence of arginine, the ArgR trimer can bind three ARG box half-sites. It also reveals the structure of the whole MtbArgR molecule itself containing both N-terminal and C-terminal domains.     

How slug herbivory of juvenile hybrid willows alters chemistry,growth and subsequent susceptibility to diverse plant enemies

Background and Aims

Selective feeding by herbivores, especially at the seedling or juvenile phase, has the potential to change plant traits and ultimately the susceptibility of surviving plants to other enemies. Moreover, since hybridization is important to speciation and can lead to introgression of traits between plant species, differential feeding (herbivore-induced mortality) can influence the expression of resistance traits of hybrids and ultimately determine the consequences of hybridization. While it would be expected that herbivore-induced mortality would lead to greater resistance, there may be trade-offs whereby resistance to one herbivore increases susceptibility to others. The hypothesis was tested that the exotic slug, Arion subfuscus, causes non-random survival of hybrid willows and alters plant: (1) susceptibility to slugs; (2) secondary and nutritional chemistry, and growth; and (3) susceptibility to other phytophages.

Methods

Two populations of plants, control and selected, were created by placing trays of juvenile willows in the field and allowing slugs access to only some. When ≤10 individuals/tray remained (approx. 85 % mortality), ‘selected’ and undamaged ‘control’ trays were returned to a common area. Traits of these populations were then examined in year 1 and in subsequent years.

Key Results

The selected population was less palatable to slugs. Surprisingly, foliar concentrations of putative defence traits (phenolic glycosides and tannins) did not differ between treatments, but the selected population had higher foliar nitrogen and protein, lower carbon to nitrogen ratio and greater above-ground biomass, indicating that vigorously growing plants were inherently more resistant to slugs. Interestingly, selected plants were more susceptible to three phytophages: an indigenous pathogen (Melampsora epitea), a native herbivorous beetle (Chrysomela knabi) and an exotic willow leaf beetle (Plagiodera versicolora).

Conclusions

This exotic slug changed the population structure of F₂ hybrid willows in unanticipated ways. Defence expression remained unchanged, while nutritional and growth traits changed. These changes caused plants to be more susceptible to other plant enemies. Other exotic herbivore species are anticipated to have similar direct and indirect effects on native plant populations.     

In silico prediction of proteins related to xyloglucan fucosyltransferases in Solanaceae genomes

Two independent studies have shown that the cell wall of pollen tubes from tobacco and tomato species contained fucosylated xyloglucan (XyG). These findings are intriguing as many reports have shown that XyG of somatic cells of these species is not fucosylated but instead is arabinosylated. In order to produce fucosylated XyG, plants must express a functional galactoside α-2-fucosyltransferase. Here, using a bioinformatics approach, we show that several candidate genes coding for XyG fucosyltransferases are present in the genome of coffee and several Solanaceae species including tomato, tobacco, potato, eggplant and pepper. BLAST and protein alignments with the 2 well-characterized XyG fucosyltransferases from Arabidopsis thaliana and Pisum sativum revealed that at least 6 proteins from different Solanaceae species and from coffee displayed the 3 conserved motifs required for XyG fucosyltransferase activity.     

Cycling of clock genes entrained to the solar rhythm enables plants to tell time: data from arabidopsis

Background and Aims An endogenous rhythm synchronized to dawn cannot time photosynthesis-linked genes to peak consistently at noon since the interval between sunrise and noon changes seasonally. In this study, a solar clock model that circumvents this limitation is proposed using two daily timing references synchronized to noon and midnight. Other rhythmic genes that are not directly linked to photosynthesis, and which peak at other times, also find an adaptive advantage in entrainment to the solar rhythm.Methods Fourteen datasets extracted from three published papers were used in a meta-analysis to examine the cyclic behaviour of the Arabidopsis thaliana photosynthesis-related gene CAB2 and the clock oscillator genes TOC1 and LHY in T cycles and N–H cycles.Key Results Changes in the rhythms of CAB2, TOC1 and LHY in plants subjected to non-24-h light:dark cycles matched the hypothesized changes in their behaviour as predicted by the solar clock model, thus validating it. The analysis further showed that TOC1 expression peaked ∼5·5 h after mid-day, CAB2 peaked close to noon, while LHY peaked ∼7·5 h after midnight, regardless of the cycle period, the photoperiod or the light:dark period ratio. The solar clock model correctly predicted the zeitgeber timing of these genes under 11 different lighting regimes comprising combinations of seven light periods, nine dark periods, four cycle periods and four light:dark period ratios. In short cycles that terminated before LHY could be expressed, the solar clock correctly predicted zeitgeber timing of its expression in the following cycle.Conclusions Regulation of gene phases by the solar clock enables the plant to tell the time, by which means a large number of genes are regulated. This facilitates the initiation of gene expression even before the arrival of sunrise, sunset or noon, thus allowing the plant to ‘anticipate’ dawn, dusk or mid-day respectively, independently of the photoperiod.     

Sensitivity of growth and biomass allocation patterns to increasing nitrogen: a comparison between ephemerals and annuals in the Gurbantunggut Desert,north-western China

Background and Aims

Biomass accumulation and allocation patterns are critical to quantifying ecosystem dynamics. However, these patterns differ among species, and they can change in response to nutrient availability even among genetically related individuals. In order to understand this complexity further, this study examined three ephemeral species (with very short vegetative growth periods) and three annual species (with significantly longer vegetative growth periods) in the Gurbantunggut Desert, north-western China, to determine their responses to different nitrogen (N) supplements under natural conditions.

Methods

Nitrogen was added to the soil at rates of 0, 0·5, 1·0, 3·0, 6·0 and 24·0 g N m⁻² year⁻¹. Plants were sampled at various intervals to measure relative growth rate and shoot and root dry mass.

Key Results

Compared with annuals, ephemerals grew more rapidly, increased shoot and root biomass with increasing N application rates and significantly decreased root/shoot ratios. Nevertheless, changes in the biomass allocation of some species (i.e. Erodium oxyrrhynchum) in response to the N treatment were largely a consequence of changes in overall plant size, which was inconsistent with an optimal partitioning model. An isometric log shoot vs. log root scaling relationship for the final biomass harvest was observed for each species and all annuals, while pooled data of three ephemerals showed an allometric scaling relationship.

Conclusions

These results indicate that ephemerals and annuals differ observably in their biomass allocation patterns in response to soil N supplements, although an isometric log shoot vs. log root scaling relationship was maintained across all species. These findings highlight that different life history strategies behave differently in response to N application even when interspecific scaling relationships remain nearly isometric.     

京津冀地区植被物候时空变化及其对城市化的响应

温度在调节植被物候的变化中起着重要作用,气候变暖和城市化均会对温度产生影响进而影响植被物候。基于京津冀地区2001—2020年的归一化植被指数(NDVI)时间序列影像,参照物候观测站点监测数据,采用动态阈值法提取出研究区关键物候参数,即生长季始期(SOG)、生长季结束期(EOG)和生长季的长度(LOG),分析近20年京津冀地区耕地、林地、草地不同植被的物候时空变化特征及其城乡差异,从而探讨植被物候对城市化的响应。结果表明：(1)耕地SOG分布呈现双波峰现象,林地和草地的SOG相对集中,3种植被的EOG的分布均较为集中;2001—2020年京津冀大部分地区的SOG提前,EOG推迟,LOG呈现延长的态势。(2)从城乡梯度物候差异的空间分布特征来看,整体而言耕地、林地、草地3种植被类型的城区物候与农村相比都存在SOG提前,EOG推迟的情况,并且城区与农村的物候差异幅度要明显大于城乡过渡带与农村的物候差异。(3)从城乡梯度物候差异的时间分布特征来看,2001—2020年间,研究区中新老城区、城乡过渡带和农村的SOG提前,EOG推迟,LOG延长,但耕地、林地、草地3种植被物候参数的城乡差异在逐年...     

A conserved dimer and global conformational changes in the structure of apo-PknE Ser/Thr protein kinase from Mycobacterium tuberculosis

The "eukaryotic-like" receptor Ser/Thr protein kinases (STPKs) are candidates for the sensors that mediate environmental adaptations of Mycobacterium tuberculosis (Mtb). To define the mechanisms of regulation and substrate recognition, we determined the crystal structure of the ligand-free, activated kinase domain (KD) of the Mtb STPK, PknE. Remarkably, the PknE KD formed a dimer similar to that first observed in the structure of the ATPgammaS complex of the Mtb paralog, PknB. This structural similarity, which occurs despite little sequence conservation between the PknB and PknE dimer interfaces, supports the idea that dimerization regulates the Mtb receptor STPKs. Insertion of the DFG motif into the ATP-binding site and other conformational differences compared the ATPgammaS:PknB complex suggest that apo-PknE is not pre-organized to bind nucleotides. This structure may represent an inactive conformation stabilized by dimerization or, alternatively, an active conformation that reveals shifts that mediate nucleotide exchange and order substrate binding.