Professional Education

  • Doctor of Philosophy, University of Connecticut, Genetics (2016)
  • Bachelor of Technology, Dr. M.G.R Educational and Research Institute, Industrial Biotechnology (2007)

Stanford Advisors


All Publications

  • Comparative Metatranscriptomics of Periodontitis Supports a Common Polymicrobial Shift in Metabolic Function and Identifies Novel Putative Disease-Associated ncRNAs. Frontiers in microbiology Ram-Mohan, N., Meyer, M. M. 2020; 11: 482


    Periodontitis is an inflammatory disease that deteriorates bone supporting teeth afflicting ∼743 million people worldwide. Bacterial communities associated with disease have been classified into red, orange, purple, blue, green, and yellow complexes based on their roles in the periodontal pocket. Previous metagenomic and metatranscriptomics analyses suggest a common shift in metabolic signatures in disease vs. healthy communities with up-regulated processes including pyruvate fermentation, histidine degradation, amino acid metabolism, TonB-dependent receptors. In this work, we examine existing metatranscriptome datasets to identify the commonly differentially expressed transcripts and potential underlying RNA regulatory mechanisms behind the metabolic shifts. Raw RNA-seq reads from three studies (including 49 healthy and 48 periodontitis samples) were assembled into transcripts de novo. Analyses revealed 859 differentially expressed (DE) transcripts, 675 more- and 174 less-expressed. Only ∼20% of the DE transcripts originate from the pathogenic red/orange complexes, and ∼50% originate from organisms unaffiliated with a complex. Comparison of expression profiles revealed variations among disease samples; while specific metabolic processes are commonly up-regulated, the underlying organisms are diverse both within and across disease associated communities. Surveying DE transcripts for known ncRNAs from the Rfam database identified a large number of tRNAs and tmRNAs as well as riboswitches (FMN, glycine, lysine, and SAM) in more prevalent transcripts and the cobalamin riboswitch in both more and less prevalent transcripts. In silico discovery identified many putative ncRNAs in DE transcripts. We report 15 such putative ncRNAs having promising covariation in the predicted secondary structure and interesting genomic context. Seven of these are antisense of ribosomal proteins that are novel and may involve maintaining ribosomal protein stoichiometry during the disease associated metabolic shift. Our findings describe the role of organisms previously unaffiliated with disease and identify the commonality in progression of disease across three metatranscriptomic studies. We find that although the communities are diverse between individuals, the switch in metabolic signatures characteristic of disease is typically achieved through the contributions of several community members. Furthermore, we identify many ncRNAs (both known and putative) which may facilitate the metabolic shifts associated with periodontitis.

    View details for DOI 10.3389/fmicb.2020.00482

    View details for PubMedID 32328037

    View details for PubMedCentralID PMC7160235

  • Regulatory context drives conservation of glycine riboswitch aptamers. PLoS computational biology Crum, M., Ram-Mohan, N., Meyer, M. M. 2019; 15 (12): e1007564


    In comparison to protein coding sequences, the impact of mutation and natural selection on the sequence and function of non-coding (ncRNA) genes is not well understood. Many ncRNA genes are narrowly distributed to only a few organisms, and appear to be rapidly evolving. Compared to protein coding sequences, there are many challenges associated with assessment of ncRNAs that are not well addressed by conventional phylogenetic approaches, including: short sequence length, lack of primary sequence conservation, and the importance of secondary structure for biological function. Riboswitches are structured ncRNAs that directly interact with small molecules to regulate gene expression in bacteria. They typically consist of a ligand-binding domain (aptamer) whose folding changes drive changes in gene expression. The glycine riboswitch is among the most well-studied due to the widespread occurrence of a tandem aptamer arrangement (tandem), wherein two homologous aptamers interact with glycine and each other to regulate gene expression. However, a significant proportion of glycine riboswitches are comprised of single aptamers (singleton). Here we use graph clustering to circumvent the limitations of traditional phylogenetic analysis when studying the relationship between the tandem and singleton glycine aptamers. Graph clustering enables a broader range of pairwise comparison measures to be used to assess aptamer similarity. Using this approach, we show that one aptamer of the tandem glycine riboswitch pair is typically much more highly conserved, and that which aptamer is conserved depends on the regulated gene. Furthermore, our analysis also reveals that singleton aptamers are more similar to either the first or second tandem aptamer, again based on the regulated gene. Taken together, our findings suggest that tandem glycine riboswitches degrade into functional singletons, with the regulated gene(s) dictating which glycine-binding aptamer is conserved.

    View details for DOI 10.1371/journal.pcbi.1007564

    View details for PubMedID 31860665

  • The Transcriptional landscape of Streptococcus pneumoniae TIGR4 reveals a complex operon architecture and abundant riboregulation critical for growth and virulence PLOS PATHOGENS Warrier, I., Ram-Mohan, N., Zhu, Z., Hazery, A., Echlin, H., Rosch, J., Meyer, M. M., van Opijnen, T. 2018; 14 (12): e1007461


    Efficient and highly organized regulation of transcription is fundamental to an organism's ability to survive, proliferate, and quickly respond to its environment. Therefore, precise mapping of transcriptional units and understanding their regulation is crucial to determining how pathogenic bacteria cause disease and how they may be inhibited. In this study, we map the transcriptional landscape of the bacterial pathogen Streptococcus pneumoniae TIGR4 by applying a combination of high-throughput RNA-sequencing techniques. We successfully map 1864 high confidence transcription termination sites (TTSs), 790 high confidence transcription start sites (TSSs) (742 primary, and 48 secondary), and 1360 low confidence TSSs (74 secondary and 1286 primary) to yield a total of 2150 TSSs. Furthermore, our study reveals a complex transcriptome wherein environment-respondent alternate transcriptional units are observed within operons stemming from internal TSSs and TTSs. Additionally, we identify many putative cis-regulatory RNA elements and riboswitches within 5'-untranslated regions (5'-UTR). By integrating TSSs and TTSs with independently collected RNA-Seq datasets from a variety of conditions, we establish the response of these regulators to changes in growth conditions and validate several of them. Furthermore, to demonstrate the importance of ribo-regulation by 5'-UTR elements for in vivo virulence, we show that the pyrR regulatory element is essential for survival, successful colonization and infection in mice suggesting that such RNA elements are potential drug targets. Importantly, we show that our approach of combining high-throughput sequencing with in vivo experiments can reconstruct a global understanding of regulation, but also pave the way for discovery of compounds that target (ribo-)regulators to mitigate virulence and antibiotic resistance.

    View details for DOI 10.1371/journal.ppat.1007461

    View details for Web of Science ID 000454721500019

    View details for PubMedID 30517198

    View details for PubMedCentralID PMC6296669

  • Analysis of the bacteriorhodopsin-producing haloarchaea reveals a core community that is stable over time in the salt crystallizers of Eilat, Israel EXTREMOPHILES Ram-Mohan, N., Oren, A., Papke, R. 2016; 20 (5): 747–57


    Stability of microbial communities can impact the ability of dispersed cells to colonize a new habitat. Saturated brines and their halophile communities are presumed to be steady state systems due to limited environmental perturbations. In this study, the bacteriorhodopsin-containing fraction of the haloarchaeal community from Eilat salt crystallizer ponds was sampled five times over 3 years. Analyses revealed the existence of a constant core as several OTUs were found repeatedly over the length of the study: OTUs comprising 52 % of the total cloned and sequenced PCR amplicons were found in every sample, and OTUs comprising 89 % of the total sequences were found in more than one, and often more than two samples. LIBSHUFF and UNIFRAC analyses showed statistical similarity between samples and Spearman's coefficient denoted significant correlations between OTU pairs, indicating non-random patterns in abundance and co-occurrence of detected OTUs. Further, changes in the detected OTUs were statistically linked to deviations in salinity. We interpret these results as indicating the existence of an ever-present core bacteriorhodopsin-containing Eilat crystallizer community that fluctuates in population densities, which are controlled by salinity rather than the extinction of some OTUs and their replacement through immigration and colonization.

    View details for DOI 10.1007/s00792-016-0864-4

    View details for Web of Science ID 000382142400015

    View details for PubMedID 27444744

  • Horizontal gene transfer, dispersal and haloarchaeal speciation. Life (Basel, Switzerland) Papke, R. T., Corral, P., Ram-Mohan, N., de la Haba, R. R., Sánchez-Porro, C., Makkay, A., Ventosa, A. 2015; 5 (2): 1405–26


    The Halobacteria are a well-studied archaeal class and numerous investigations are showing how their diversity is distributed amongst genomes and geographic locations. Evidence indicates that recombination between species continuously facilitates the arrival of new genes, and within species, it is frequent enough to spread acquired genes amongst all individuals in the population. To create permanent independent diversity and generate new species, barriers to recombination are probably required. The data support an interpretation that rates of evolution (e.g., horizontal gene transfer and mutation) are faster at creating geographically localized variation than dispersal and invasion are at homogenizing genetic differences between locations. Therefore, we suggest that recurrent episodes of dispersal followed by variable periods of endemism break the homogenizing forces of intrapopulation recombination and that this process might be the principal stimulus leading to divergence and speciation in Halobacteria.

    View details for DOI 10.3390/life5021405

    View details for PubMedID 25997110

    View details for PubMedCentralID PMC4500145

  • Evidence from phylogenetic and genorne fingerprinting analyses suggests rapidly changing variation in Halorubrum and Haloarcula populations FRONTIERS IN MICROBIOLOGY Mohan, N., Fullmer, M. S., Makkay, A. M., Wheeler, R., Ventosa, A., Naor, A., Gogarten, J., Papke, R. 2014; 5: 143


    Halobacteria require high NaCl concentrations for growth and are the dominant inhabitants of hypersaline environments above 15% NaCl. They are well-documented to be highly recombinogenic, both in frequency and in the range of exchange partners. In this study, we examine the genetic and genomic variation of cultured, naturally co-occurring environmental populations of Halobacteria. Sequence data from multiple loci (~2500 bp) identified many closely and more distantly related strains belonging to the genera Halorubrum and Haloarcula. Genome fingerprinting using a random priming PCR amplification method to analyze these isolates revealed diverse banding patterns across each of the genera and surprisingly even for isolates that are identical at the nucleotide level for five protein coding sequenced loci. This variance in genome structure even between identical multilocus sequence analysis (MLSA) haplotypes indicates that accumulation of genomic variation is rapid: faster than the rate of third codon substitutions.

    View details for DOI 10.3389/fmicb.2014.00143

    View details for Web of Science ID 000334277900001

    View details for PubMedID 24782838

    View details for PubMedCentralID PMC3988388

  • Cell sorting analysis of geographically separated hypersaline environments EXTREMOPHILES Zhaxybayeva, O., Stepanauskas, R., Mohan, N., Papke, R. 2013; 17 (2): 265–75


    Biogeography of microbial populations remains to be poorly understood, and a novel technique of single cell sorting promises a new level of resolution for microbial diversity studies. Using single cell sorting, we compared saturated NaCl brine environments (32-35 %) of the South Bay Salt Works in Chula Vista in California (USA) and Santa Pola saltern near Alicante (Spain). Although some overlap in community composition was detected, both samples were significantly different and included previously undiscovered 16S rRNA sequences. The community from Chula Vista saltern had a large bacterial fraction, which consisted of diverse Bacteroidetes and Proteobacteria. In contrast, Archaea dominated Santa Pola's community and its bacterial fraction consisted of the previously known Salinibacter lineages. The recently reported group of halophilic Archaea, Nanohaloarchaea, was detected at both sites. We demonstrate that cell sorting is a useful technique for analysis of halophilic microbial communities, and is capable of identifying yet unknown or divergent lineages. Furthermore, we argue that observed differences in community composition reflect restricted dispersal between sites, a likely mechanism for diversification of halophilic microorganisms.

    View details for DOI 10.1007/s00792-013-0514-z

    View details for Web of Science ID 000315574600007

    View details for PubMedID 23358730

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