Biology subjects

Mitra, A.

Publications and source records attributed to Mitra, A..

4 recordsLinked to original sources

bioRxiv2017

Integrated analysis of DSB patterns reveals precisely DSB formation mechanisms following replication fork collapse

DNA double-strand breaks (DSBs) can be detected by label-based sequencing or pulsed-field gel electrophoresis (PFGE). Sequencing yields population-average DSB frequencies genome-wide, while PFGE reveals percentages of broken chromosomes. We constructed a mathematical framework to combine advantages of both: high-resolution DSB locations and their population distribution. We also use sequencing read patterns to identify replication-induced DSBs and active replication origins. We describe changes in spatiotemporal replication program upon hydroxyurea-induced replication stress. We found that one-ended DSBs, resulting from collapsed replication forks, are population-representative, while majority of two-ended DSBs (79-100%) are not. To study replication fork collapse, we used strains lacking the checkpoint protein Mec1 and the endonuclease Mus81 and quantified that 19% and 13% of hydroxyurea-induced one-ended DSBs are Mec1-and Mus81-dependent, respectively. We also clarified that Mus81-induced one-ended DSBs are Mec1-dependent.

genomics↗

bioRxiv2017

Reverse Watson-Crick purine-purine base pairs : the Sharp-turn motif and other structural consequences in functional RNAs

Identification of static and/or dynamic roles of different noncanonical base pairs is essential for a comprehensive understanding of the sequence-structure-function space of RNA. In this context, reverse Watson-Crick purine-purine base pairs (A:A, G:G&A:GW:W Trans) constitute an interesting class of noncanonical base pairs in RNA due to their characteristic C1'-C1' distance (highest among all base pairing geometries) and parallel local strand orientation. Structural alignment of the RNA stretches containing these W:W Trans base pairs with their corresponding homologous sites in a non-redundant set of RNA crystal structures show that, as expected, these base pairs are associated with specific structural folds or functional roles. Detailed analysis of these contexts further revealed a bimodal distribution in the local backbone geometry parameters associated with these base pairs. One mode, populated by both A:A and G:G W:W Trans pairs, manifests itself as a characteristic backbone fold. We call this fold a Sharp-turn motif. The other mode is exclusively associated with A:A W:W Trans pairs involved in mediating higher order interactions. The same trend is also observed in available solution NMR structures. We have also characterized the importance of recurrent hydrogen bonding interactions between adenine and guanine in W:W Trans geometry. Quantum chemical calculations performed at M05-2X/6-31++(2d,2p) level explain how the characteristic electronic properties of these W:W Trans base pairs facilitate their occurrence in such exclusive structural folds that are important for RNA functionalities.

biophysics↗

bioRxiv2017

Structural landscape of base pairs containing post-transcriptional modifications in RNA

Base pairs involving post-transcriptionally modified nucleobases are believed to play important roles in a wide variety of functional RNAs. Here we present our attempts towards understanding the structural and functional role of naturally occurring modified base pairs by analyzing their distribution in different RNA classes, with the help of crystal structure and sequence database analyses. In addition, we quantify the variation in geometrical features of modified base pairs within RNA structures, and characterize their optimum geometries and binding energies using advanced quantum chemical methods. Further comparison of modified base pairs with their unmodified counterparts illustrates the effect of steric and electronic structure alterations due to base modifications. Analysis of specific structural contexts of modified base pairs in RNA crystal structures revealed several interesting scenarios, including those at the tRNA:rRNA interface, antibiotic-binding site and the three-way junctions within tRNA, which when analyzed in context of available experimental data, allowed us to correlate the occurrence and strength of modified base pairs with the specific functional roles they play in context of RNA macromolecules.

biophysics↗

bioRxiv2017

How does Mg2+ modulate the RNA folding mechanism - a case study of G:C W:W Trans base pair.

Reverse Watson-Crick G:C base pairs (G:C W:W Trans) occur frequently in different functional RNAs. It is one of the few base pairs whose gas phase optimized isolated geometry is inconsistent with the corresponding experimental geometry. Several earlier studies indicate that accumulation of positive charge near N7 of guanine, through posttranscriptional modification, direct protonation or coordination with Mg2+, can stabilize the experimental geometry. Interestingly, recent studies reveal significant variation in the position of putatively bound Mg2+. This, in conjunction with recently raised doubts regarding some of the Mg2+ assignments near the imino nitrogen of guanine, is suggestive of the existence of multiple Mg2+ binding modes for this base pair. Our detailed investigation of Mg2+ bound G:C W:W Trans pairs, occurring in high resolution RNA crystal structures, show that they occur in 14 different contexts, 8 out of which display Mg2+ binding at the Hoogsteen edge of guanine. Further examination of occurrences in these 8 contexts led to the characterization of three different Mg2+ binding modes, (i) direct binding via N7 coordination, (ii) direct binding via O6 coordination and (iii) binding via hydrogen bonding interaction with the first shell water molecules. In the crystal structures, the latter two modes are associated with a buckled and propeller twisted geometry of the base pair. Interestingly, respective optimized geometries of these different Mg2+ binding modes (optimized at B3LYP) are consistent with their corresponding experimental geometries. Subsequent interaction energy calculations at MP2 level, and decomposition of its components, suggest that for G:C W:W Trans, Mg2+ binding can fine tune the base pair geometries without compromising with their stability. Our results, therefore, underline the importance of the mode of binding of Mg2+ ions in shaping RNA structure, folding and function.

biophysics↗