Interplay between Top1 and Mms21/Nse2 mediated sumoylation in stable maintenance of long chromosomes

Genetic information in cells is encrypted in DNA molecules forming chromosomes of varying sizes. Accurate replication and partitioning of chromosomes in the crowded cellular milieu is a complex process involving duplication, folding and movement. Longer chromosomes may be more susceptible to mis-segregation or DNA damage and there may exist specialized physiological mechanisms preventing this. Here, we present genetic evidence for such a mechanism which depends on Mms21/Nse2 mediated sumoylation and topoisomerase-1 (Top1) for maintaining stability of longer chromosomes. While mutations inactivating Top1 or the SUMO ligase activity of Mms21 (mms21sl) individually destabilized yeast artificial chromosomes (YACs) to a modest extent, the mms21sl top1 double mutant exhibited a synthetic-sick phenotype, and showed preferential destabilization of the longer chromosome relative to shorter chromosomes. In contrast, an smc6-56 top1 mutant defective in Smc6, another subunit of the Smc5/6 complex, of which Mms21 is a component, did not show such a preferential enhancement in frequency of loss of the longer YAC, indicating that this defect may be specific to the deficiency in SUMO ligase activity of Mms21 in the mms21sl top1 mutants. In addition, mms21sl top1 double mutants harboring a longer fusion derivative of natural yeast chromosomes IV and XII displayed reduced viability, consistent with enhanced chromosome instability, relative to single mutants or the double mutant having the natural (shorter) non-fused chromosomes. Our findings reveal a functional interplay between Mms21 and Top1 in maintenance of longer chromosomes, and suggest that lack of sumoylation of Mms21 targets coupled with Top1 deficiency is a crucial requirement for accurate inheritance of longer chromosomes

Limiting the Extent of the RDN1 Heterochromatin Domain by a Silencing Barrier and Sir2 Protein Levels in Saccharomyces cerevisiae▿

In Saccharomyces cerevisiae, transcriptional silencing occurs at the cryptic mating-type loci (HML and HMR), telomeres, and ribosomal DNA (rDNA; RDN1). Silencing in the rDNA is unusual in that polymerase II (Pol II) promoters within RDN1 are repressed by Sir2 but not Sir3 or Sir4. rDNA silencing unidirectionally spreads leftward, but the mechanism of limiting its spreading is unclear. We searched for silencing barriers flanking the left end of RDN1 by using an established assay for detecting barriers to HMR silencing. Unexpectedly, the unique sequence immediately adjacent to RDN1, which overlaps a prominent cohesin binding site (CARL2), did not have appreciable barrier activity. Instead, a fragment located 2.4 kb to the left, containing a tRNAGln gene and the Ty1 long terminal repeat, had robust barrier activity. The barrier activity was dependent on Pol III transcription of tRNAGln, the cohesin protein Smc1, and the SAS1 and Gcn5 histone acetyltransferases. The location of the barrier correlates with the detectable limit of rDNA silencing when SIR2 is overexpressed, where it blocks the spreading of rDNA heterochromatin. We propose a model in which normal Sir2 activity results in termination of silencing near the physical rDNA boundary, while tRNAGln blocks silencing from spreading too far when nucleolar Sir2 pools become elevated

Arsenic tolerances in rice (Oryza sativa) have a predominant role in transcriptional regulation of a set of genes including sulphur assimilation pathway and antioxidant system

World wide arsenic (As) contamination of rice has raised much concern as it is the staple crop for millions. Four most commonly cultivated rice cultivars, Triguna, IR-36, PNR-519 and IET-4786, of the West Bengal region were taken for a hydroponic study to examine the effect of arsenate (AsV) and arsenite (AsIII) on growth response, expression of genes and antioxidants vis-a-vis As accumulation. The rice genotypes responded differentially under AsV and AsIII stress in terms of gene expression and antioxidant defences. Some of the transporters were up-regulated in all rice cultivars at lower doses of As species, except IET-4786. Phytochelatin synthase, GST and γ-ECS showed considerable variation in their expression pattern in all genotypes, however in IET-4786 they were generally down-regulated in higher AsIII stress. Similarly, most of antioxidants such as superoxide dismutase (SOD), ascorbate peroxidase (APX), guaiacol peroxidase (GPX), catalase (CAT), monodehydroascorbate reductase (MDHAR) and dehydroascorbate reductase (DHAR) increased significantly in Triguna, IR-36 and PNR-519 and decreased in IET-4786. Our study suggests that Triguna, IR-36 and PNR-519 are tolerant rice cultivars accumulating higher arsenic; however IET-4786 is susceptible to As-stress and accumulates less arsenic than other cultivars

Managing aquifer recharge and sustaining groundwater use : developing a capacity building program for creating local groundwater champions

Participatory groundwater management is increasingly being recognised for its ability to address the challenges of equity, efficiency and sustainability. It can particularly help with effective engagement at the grassroots level for monitoring, recharging and managing the groundwater as a common pool resource. The main aim of this article is to discuss the training and management process used and the lessons learnt from a participatory groundwater management project, titled Managing Aquifer Recharge and Sustainable Groundwater Use through Village-level Intervention (MARVI). In this project, researchers, rural development facilitators and local villagers worked together to initiate participatory groundwater monitoring in 11 villages from the Dharta and Meghraj watersheds in Rajasthan and Gujarat, India. The study involved educating villagers through an intensive program of capacity building, wherein the villagers who participated in the program were called Bhujal Jaankars (BJs), a Hindi word meaning ‘groundwater informed’. The BJs were trained in their local settings through relevant theory and practical exercises, so that they could perform a geo-hydrological evaluation of their area, monitor groundwater and share their findings and experiences with their village community. The study has highlighted that with a well-designed program of capacity building and on-going support through training and nurturing, BJs can play an important role in monitoring watertable depth and other data for estimating groundwater recharge, leading to a sharing of the groundwater information with the local village community to influence the sustainable use of groundwater. Overall they can act as local champions for groundwater futures. Further, this study has demonstrated that BJ capacity building can help to provide a scientific basis for village level groundwater dialogue and assist village communities and other stakeholders to improve their decision making regarding groundwater use, crop selection, agronomy, recharge strategies and other aspects of sustainable groundwater management. Although the BJ program has been successful and BJs can act as a valuable interface between local communities and other stakeholders managed aquifer recharge activities, there still exists some challenges to the BJ programme, such as the need for mechanisms and funding sources that will sustain the BJs over the longer term; wider acceptance of BJs among scientific communities and policy makers; and the acceptance of the role and involvements of BJs in natural resources management programs of the State and Central governments in India