Biology subjects

Portella, G.

Publications and source records attributed to Portella, G..

4 recordsLinked to original sources

bioRxiv2024

Systematic study of hybrid triplex topology and stability suggests a general triplex-mediated regulatory mechanism

By combining in-silico, biophysical and in-cellulo experiments, we decipher the topology, physical and potential biological properties of hybrid-parallel nucleic acids triplexes; an elusive structure at the basis of life. We found that hybrid triplex topology follows a stability order: r(Py)-d(Pu){middle dot}r(Py)> r(Py)-d(Pu){middle dot}d(Py)> d(Py)-d(Pu){middle dot}d(Py)> d(Py)-d(Pu){middle dot}r(Py). The r(Py)-d(Pu){middle dot}d(Py) triplex is expected to be the preferred in the cell as it avoids the need to open the duplex reducing the torsional stress required for triplex formation in the r(Py)-d(Pu){middle dot}r(Py) topology. Upon a massive collection of melting data, we have created the first predictor for hybrid triplex stability. Leveraging this predictor, we conducted a comprehensive scan to assess the likelihood of the human genome and transcriptome to engage in triplex formation. Our findings unveil a remarkable inclination - of both the human genome and transcriptome - to generate hybrid triplex formation, particularly within untranslated (UTRs) and regulatory regions, thereby corroborating the existence of a triplex-mediated regulatory mechanism. Furthermore, we found a correlation between nucleosome linkers and TFS which agree with a putative role of triplexes in arranging chromatin structure and local/global level.

molecular biology↗

bioRxiv2020

THE IMPACT OF THE HYDROXYMETHYLCYTOSINE EPIGENETIC SIGNATURE ON DNA STRUCTURE AND FUNCTION

We present a comprehensive, experimental and theoretical study of the impact of 5-hydroxymethylation of DNA cytosine. Using molecular dynamics, biophysical experiments and NMR spectroscopy, we found that Ten-Eleven translocation (TET) dioxygenases generate an epigenetic variant with structural and physical properties not too different to those of 5-methylcytosine. Experiments and simulations demonstrate that 5-methyl-cytosine (mC) and 5-hydroxymethyl-cytosine (hmC) generally lead to more rigid duplexes with poorer circularization efficiencies and lower ability to form nucleosomes. In particular, we can rule out the hypothesis that hydroxymethylation reverts to unmodified cytosine physical properties, as hmC is even more rigid than mC. Thus, we do not expect dramatic changes in the chromatin structure induced by differences in physical properties between d(mCpG) and d(hmCpG). On the contrary, our simulations suggest that methylated-DNA binding domains (MBD), associated with repression activities, are very sensitive to the substitution d(mCpG)[->] d(hmCpG), while MBD3 which has a dual activation/repression activity is not sensitive to the d(mCpG) [->] d(hmCpG) change. Overall, while changes in gene activity due to cytosine methylation are the result of the combination of stiffness-related chromatin reorganization and MBD binding, those associated to 5-hydroxylation of methylcytosine could be explained by a change in the balance of repression/activation pathways related to differential MBD binding.

biochemistry↗

bioRxiv2017

5-Formylcytosine controls nucleosome positioning through covalent histone-DNA interaction

Nucleosomes are the basic unit of chromatin that ensure genome integrity and control access to the genetic information. The organization of nucleosomes is influenced by the underlying DNA sequence itself, transcription factors or other transcriptional machinery associated proteins and chromatin remodeling complexes (1-4). Herein, we show that the naturally occurring DNA modification, 5-formylcytosine (5fC) contributes to the positioning of nucleosomes. We show that the ability of 5fC to position nucleosomes in vitro is associated with the formation of covalent interactions between histone residues and 5fC in the form of Schiff bases. We demonstrate that similar interactions can occur in a cellular environment and define their specific genomic loci in mouse embryonic stem cells. Collectively, our findings identify 5fC as a determinant of nucleosomal organization in which 5fC plays a role in establishing distinct regulatory regions that are linked to gene expression Our study provides a previously unknown molecular mechanism, involving the formation of reversible-covalent bonds between chromatin and DNA that supports a molecular linkage between DNA sequence, DNA base modification and chromatin structure.

genomics↗

bioRxiv2017

RNA G-quadruplexes mark repressive upstream open reading frames in human mRNAs

RNA secondary structures in the 5 untranslated regions (UTRs) of mRNAs have been characterised as key determinants of translation initiation. However the role of non-canonical secondary structures, such as RNA G-quadruplexes (rG4s), in modulating translation of human mRNAs and the associated mechanisms remain largely unappreciated. Here we use a ribosome profiling strategy to investigate the translational landscape of human mRNAs with structured 5 untranslated regions (5-UTR). We found that inefficiently translated mRNAs, containing rG4-forming sequences in their 5-UTRs, have an accumulation of ribosome footprints in their 5-UTRs. We show that rG4-forming sequences are determinants of 5-UTR translation, suggesting that the folding of rG4 structures thwarts the translation of protein coding sequences (CDS) by stimulating the translation of repressive upstream open reading frames (uORFs). To support our model, we demonstrate that depletion of two rG4s-specialised DEAH-box helicases, DHX36 and DHX9, shifts translation towards rG4-containing uORFs reducing the translation of selected transcripts comprising proto-oncogenes, transcription factors and epigenetic regulators. Transcriptome-wide identification of DHX9 binding sites using individual-nucleotide resolution UV crosslinking and immunoprecipitation (iCLIP) demonstrate that translation regulation is mediated through direct physical interaction between the helicase and its rG4 substrate. Our findings unveil a previously unknown role for non-canonical structures in governing 5-UTR translation and suggest that the interaction of helicases with rG4s could be considered as a target for future therapeutic intervention.

genomics↗