Homology search and DNA strand-exchange reactions are central to homologous recombination in meiosis. During meiosis, these processes are regulated such that the probability of choosing a homolog ...chromatid as recombination partner is enhanced relative to that of choosing a sister chromatid. This regulatory process occurs as homologous chromosomes pair in preparation for assembly of the synaptonemal complex. Two strand-exchange proteins, Rad51 and Dmc1, cooperate in regulated homology search and strand exchange in most organisms. Here, we summarize studies on the properties of these two proteins and their accessory factors. In addition, we review current models for the assembly of meiotic strand-exchange complexes and the possible mechanisms through which the interhomolog bias of recombination partner choice is achieved.
The central recombination enzyme RAD51 has been implicated in replication fork processing and restart in response to replication stress. Here, we use a separation-of-function allele of RAD51 that ...retains DNA binding, but not D-loop activity, to reveal mechanistic aspects of RAD51's roles in the response to replication stress. Here, we find that cells lacking RAD51's enzymatic activity protect replication forks from MRE11-dependent degradation, as expected from previous studies. Unexpectedly, we find that RAD51's strand exchange activity is not required to convert stalled forks to a form that can be degraded by DNA2. Such conversion was shown previously to require replication fork regression, supporting a model in which fork regression depends on a non-enzymatic function of RAD51. We also show RAD51 promotes replication restart by both strand exchange-dependent and strand exchange-independent mechanisms.
The Eukaryotic RecA-like proteins Rad51 and Dmc1 cooperate during meiosis to promote recombination between homologous chromosomes by repairing programmed DNA double strand breaks (DSBs). Previous ...studies showed that Rad51 and Dmc1 form partially overlapping co-foci. Here we show these Rad51-Dmc1 co-foci are often arranged in pairs separated by distances of up to 400 nm. Paired co-foci remain prevalent when DSBs are dramatically reduced or when strand exchange or synapsis is blocked. Super-resolution dSTORM microscopy reveals that individual foci observed by conventional light microscopy are often composed of two or more substructures. The data support a model in which the two tracts of ssDNA formed by a single DSB separate from one another by distances of up to 400 nm, with both tracts often bound by one or more short (about 100 nt) Rad51 filaments and also by one or more short Dmc1 filaments.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
During meiosis, repair of programmed DNA double-strand breaks (DSBs) by recombination promotes pairing of homologous chromosomes and their connection by crossovers. Two DNA strand-exchange proteins, ...Rad51 and Dmc1, are required for meiotic recombination in many organisms. Studies in budding yeast imply that Rad51 acts to regulate Dmc1's strand exchange activity, while its own exchange activity is inhibited. However, in a dmc1 mutant, elimination of inhibitory factor, Hed1, activates Rad51's strand exchange activity and results in high levels of recombination without participation of Dmc1. Here we show that Rad51-mediated meiotic recombination is not subject to regulatory processes associated with high-fidelity chromosome segregation. These include homolog bias, a process that directs strand exchange between homologs rather than sister chromatids. Furthermore, activation of Rad51 does not effectively substitute for Dmc1's chromosome pairing activity, nor does it ensure formation of the obligate crossovers required for accurate homolog segregation. We further show that Dmc1's dominance in promoting strand exchange between homologs involves repression of Rad51's strand-exchange activity. This function of Dmc1 is independent of Hed1, but requires the meiotic kinase, Mek1. Hed1 makes a relatively minor contribution to homolog bias, but nonetheless this is important for normal morphogenesis of synaptonemal complexes and efficient crossing-over especially when DSB numbers are decreased. Super-resolution microscopy shows that Dmc1 also acts to organize discrete complexes of a Mek1 partner protein, Red1, into clusters along lateral elements of synaptonemal complexes; this activity may also contribute to homolog bias. Finally, we show that when interhomolog bias is defective, recombination is buffered by two feedback processes, one that increases the fraction of events that yields crossovers, and a second that we propose involves additional DSB formation in response to defective homolog interactions. Thus, robust crossover homeostasis is conferred by integrated regulation at initiation, strand-exchange and maturation steps of meiotic recombination.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract
Certification systems for sustainable agricultural commodities typically rely on auditors to verify that producers comply with environmental, social, and legal standards. In the oil palm ...industry, the Roundtable on Sustainable Palm Oil (RSPO) certification system promises to address core sectoral sustainability concerns—including deforestation, fire, biodiversity loss, peatland drainage, forced labor, and land tenure conflict—by applying third-party audits to large-scale oil palm growers. Audits are designed to detect noncompliances with the standard, and to ensure that growers resolve these nonconformities. Yet, the role of RSPO audits in generating additionality across key sustainability issues remains unclear. Here, we compiled and analyzed data from a timeseries of annual audit reports for two-thirds of all certified oil palm growers in Indonesia as of December 2015 (
n
= 114 certified growers and 262 reports). We found that certified growers were required to address a median of four noncompliances per audit (range 0–37), with more noncompliances detected at initial certification than during subsequent audits. Certification demanded the most changes under our thematic areas of Waste & Pollution (9% of all noncompliances) and Employment (7%) and the least changes in Fire (<1%) and Corporate Social Responsibility programs (<1%). Thus, while many RSPO certified growers make real changes to achieve certification, these changes do not always address core sectoral performance concerns. Regression analysis indicated that RSPO member and certification body were significantly correlated with noncompliance frequency. This suggests that each member has a different cost of compliance and indicates heterogeneous stringency of standard application by certification bodies. We recommend that future research evaluate how post-2015 changes in RSPO assurance processes have interacted with a more robust and comprehensive certification standard to alter the additionality from third-party audits.
Meiotic recombination in budding yeast requires two RecA-related proteins, Rad51 and Dmc1, both of which form filaments on DNA capable of directing homology search and catalyzing formation of ...homologous joint molecules (JMs) and strand exchange. With use of a separation-of-function mutant form of Rad51 that retains filament-forming but not JM-forming activity, we show that the JM activity of Rad51 is fully dispensable for meiotic recombination. The corresponding mutation in Dmc1 causes a profound recombination defect, demonstrating Dmc1's JM activity alone is responsible for meiotic recombination. We further provide biochemical evidence that Rad51 acts with Mei5-Sae3 as a Dmc1 accessory factor. Thus, Rad51 is a multifunctional protein that catalyzes recombination directly in mitosis and indirectly, via Dmc1, during meiosis.
Abstract
Stream CO
2
emissions contribute significantly to atmospheric climate forcing. While there are strong indications that groundwater inputs sustain these emissions, the specific biogeochemical ...pathways and timescales involved in this lateral CO
2
export are still obscure. Here, via an extensive radiocarbon (
14
C) characterisation of CO
2
and DOC in stream water and its groundwater sources in an old-growth boreal forest, we demonstrate that the
14
C-CO
2
is consistently in tune with the current atmospheric
14
C-CO
2
level and shows little association with the
14
C-DOC in the same waters. Our findings thus indicate that stream CO
2
emissions act as a shortcut that returns CO
2
recently fixed by the forest vegetation to the atmosphere. Our results expose a positive feedback mechanism within the C budget of forested catchments, where stream CO
2
emissions will be highly sensitive to changes in forest C allocation patterns associated with climate and land-use changes.
► Meta-analysis including 107 independent studies. ► Concentrations of dominant base cations increase in soil solution and stream water following N additions. ► Base cations decrease in bulk soil and ...plant foliage following N additions but only among studies shorter than five years. ► No significant changes in foliar base cation concentrations among studies five years or longer. ► No evidence that N additions have had large-scale detrimental effect on base cation availability.
The dominant base cations (BC; i.e., Ca
2+, Mg
2+, K
+, and Na
+) are important in buffering soil and water acidity in both terrestrial and aquatic ecosystems. Ca
2+, Mg
2+, and K
+ are also important in many plant physiological functions. Because BC availability is affected by changes in the nitrogen (N) cycle, we conducted a meta-analysis of previously published data to determine if N fertilization alters the availability of BC in terrestrial and stream ecosystems across biomes. We include data from 107 independent studies published in 62 different articles, taking a holistic perspective on BC by examining their responses to added N in plant foliage, bulk soil, soil solution, and stream water. Our results suggest N fertilization may accelerate BC loss from terrestrial ecosystems over time periods less than five years. We found that N additions resulted in an overall 24% decrease in the availability of exchangeable Ca
2+, Mg
2+, and K
+ in the bulk soil of boreal forest, temperate forest, and grassland biomes. Collectively, responses of BC in boreal forest, temperate forest, tropical forest, and grassland biomes increased following N fertilization by about 71% in soil solution and 48% in stream waters. Additionally, BC responses in foliage decreased in boreal forest and temperate forest biomes following N additions over time periods less than five years, but there were no significant changes over longer time periods. Despite large short-term shifts in BC responses following N additions, we did not find evidence of widespread negative impacts on ecosystems over time periods greater than five years. This analysis suggests effects of N addition on the availability of exchangeable BC may diminish over time. Although the effects on BC can be substantial over periods less than five years, there is little available evidence that N fertilization has had large-scale detrimental effects on the availability of BC needed for plant growth within terrestrial or aquatic ecosystems.
Here we show that labile particulate iron and manganese concentrations in the upper 500 m of the Western Subarctic Pacific, an iron‐limited High Nutrient Low Chlorophyll (HNLC) region, have prominent ...subsurface maxima between 100–200 m, reaching 3 nM and 600 pM, respectively. The subsurface concentration maxima in particulate Fe are characterized by a more reduced oxidation state, suggesting a source from primary volcagenic minerals such as from the Kuril/Kamchatka margin. The systematics of these profiles suggest a consistently strong lateral advection of labile Mn and Fe from redox‐mobilized labile sources at the continental shelf supplemented by a more variable source of Fe from the upper continental slope. This subsurface supply of iron from the continental margin is shallow enough to be accessible to the surface through winter upwelling and vertical mixing, and is likely a key source of bioavailable Fe to the HNLC North Pacific.
During meiosis, homologous recombination repairs programmed DNA double-stranded breaks. Meiotic recombination physically links the homologous chromosomes ("homologs"), creating the tension between ...them that is required for their segregation. The central recombinase in this process is Dmc1. Dmc1's activity is regulated by its accessory factors including the heterodimeric protein Mei5-Sae3 and Rad51. We use a gain-of-function dmc1 mutant, dmc1-E157D, that bypasses Mei5-Sae3 to gain insight into the role of this accessory factor and its relationship to mitotic recombinase Rad51, which also functions as a Dmc1 accessory protein during meiosis. We find that Mei5-Sae3 has a role in filament formation and stability, but not in the bias of recombination partner choice that favors homolog over sister chromatids. Analysis of meiotic recombination intermediates suggests that Mei5-Sae3 and Rad51 function independently in promoting filament stability. In spite of its ability to load onto single-stranded DNA and carry out recombination in the absence of Mei5-Sae3, recombination promoted by the Dmc1 mutant is abnormal in that it forms foci in the absence of DNA breaks, displays unusually high levels of multi-chromatid and intersister joint molecule intermediates, as well as high levels of ectopic recombination products. We use super-resolution microscopy to show that the mutant protein forms longer foci than those formed by wild-type Dmc1. Our data support a model in which longer filaments are more prone to engage in aberrant recombination events, suggesting that filament lengths are normally limited by a regulatory mechanism that functions to prevent recombination-mediated genome rearrangements.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
PDF
