A practical overview of molecular replacement: Clostridioides difficile PilA1, a difficult case study

Modes and model building in SHELXE

Density modification is a standard step to provide a route for routine structure solution by any experimental phasing method, with single-wavelength or multi-wavelength anomalous diffraction being the most popular methods, as well as to extend fragments or incomplete models into a full solution. The effect of density modification on the starting maps from either source is illustrated in the case of SHELXE. The different modes in which the program can run are reviewed; these include less well known uses such as reading external phase values and weights or phase distributions encoded in Hendrickson-Lattman coefficients. Typically in SHELXE, initial phases are calculated from experimental data, from a partial model or map, or from a combination of both sources. The initial phase set is improved and extended by density modification and, if the resolution of the data and the type of structure permits, polyalanine tracing. As a feature to systematically eliminate model bias from phases derived from predicted models, the trace can be set to exclude the area occupied by the starting model. The trace now includes an extension into the gamma position or hydrophobic and aromatic side chains if a sequence is provided, which is performed in every tracing cycle. Once a correlation coefficient of over 30% between the structure factors calculated from such a trace and the native data indicates that the structure has been solved, the sequence is docked in all model-building cycles and side chains are fitted if the map supports it. The extensions to the tracing algorithm brought in to provide a complete model are discussed. The improvement in phasing performance is assessed using a set of tests.

Clostridioides difficile infection: traversing host-pathogen interactions in the gut

C. difficile is the primary cause for nosocomial infective diarrhoea. For a successful infection, C. difficile must navigate between resident gut bacteria and the harsh host environment. The perturbation of the intestinal microbiota by broad-spectrum antibiotics alters the composition and the geography of the gut microbiota, deterring colonization resistance, and enabling C. difficile to colonize. This review will discuss how C. difficile interacts with and exploits the microbiota and the host epithelium to infect and persist. We provide an overview of C. difficile virulence factors and their interactions with the gut to aid adhesion, cause epithelial damage and mediate persistence. Finally, we document the host responses to C. difficile, describing the immune cells and host pathways that are associated and triggered during C. difficile infection.

Novel Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV2) and Other Coronaviruses: A Genome-wide Comparative Annotation and Analysis

Novel strain of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV2) causes mild to severe respiratory illness. The early symptoms may be fever, dry cough, sour throat, and difficulty in breathing which may lead to death in severe cases. Compared to previous outbreaks like SARS-CoV and Middle East Respiratory Syndrome (MERS), SARS-CoV2 disease (COVID-19) outbreak has been much distressing due to its high rate of infection but low infection fatality rate (IFR) with 1.4% around the world. World Health Organization (WHO) has declared (COVID-19) a pandemic on March 11, 2020. In the month of January 2020, the whole genome of SARS-CoV2 was sequenced which made work easy for researchers to develop diagnostic kits and to carry out drug repurposing to effectively alleviate the pandemic situation in the world. Now, it is important to understand why this virus has high rate of infectivity or is there any factor involved at the genome level which actually facilitates this virus infection globally? In this study, we have extensively analyzed the whole genomes of different coronaviruses infecting humans and animals in different geographical locations around the world. The main aim of the study is to identify the similarity and the mutational adaptation of the coronaviruses from different host and geographical locations to the SARS-CoV2 and provide a better strategy to understand the mutational rate for specific target-based drug designing. This study is focused to every annotation in a comparative manner which includes SNPs, repeat analysis with the different categorization of the short-sequence repeats and long-sequence repeats, different UTR’s, transcriptional factors, and the predicted matured peptides with the specific length and positions on the genomes. The extensive analysis on SNPs revealed that Wuhan SARS-CoV2 and Indian SARS-CoV2 are having only eight SNPs. Collectively, phylogenetic analysis, repeat analysis, and the polymorphism revealed the genomic conserveness within the SARS-CoV2 and few other coronaviruses with very less mutational chances and the huge distance and mutations from the few other species.