SummarySpo0J and Soj belong to the ParB/ParA family of proteins involved in chromosome and plasmid segregation in bacteria. In Bacillus subtilis, Spo0J protein binds to several specific sites, parS, located on both sides of the origin of DNA replication, oriC, and apparently self-associates to form large discrete foci visible by fluorescence microscopy. Soj protein forms large 'patches' probably associated with the nucleoid, which can undergo dynamic, co-operative jumping from nucleoid to nucleoid in the presence of Spo0J. Patches of Soj protein somehow help to bring about the condensation of Spo0J foci. Soj is also a negative regulator of transcription. In the absence of Spo0J, Soj is statically distributed on each of the nucleoids in the cell and blocks the transcription of several sporulation genes. To analyse the functional interaction between Spo0J and Soj further, we have constructed and studied a collection of spo0J mutants. Most of the mutants completely prevent Spo0J from interacting with DNA. One mutation impairs the formation of compact Spo0J foci and simultaneously results in loss of Soj movement. We also isolated one spo0J mutant, in which the frequency of Soj internucleoid oscillation is highly increased. Both mutations affecting the interaction with Soj lie in the N-terminal coding part of spo0J, whereas the substitutions affecting DNA binding lie in the mid-to C-terminal coding region.

Summary Mitochondrial fatty acid synthesis (mtFAS) shares acetyl-CoA with the Krebs cycle as a common substrate and is required for the production of octanoic acid (C8) precursors of lipoic acid (LA) in mitochondria. MtFAS is a conserved pathway essential for respiration. In a genetic screen in Saccharomyces cerevisiae designed to further elucidate the physiological role of mtFAS, we isolated mutants with defects in mitochondrial post-translational gene expression processes, indicating a novel link to mitochondrial gene expression and respiratory chain biogenesis. In our ensuing analysis, we show that mtFAS, but not lipoylation per se, is required for respiratory competence. We demonstrate that mtFAS is required for mRNA splicing, mitochondrial translation and respiratory complex assembly, and provide evidence that not LA per se, but fatty acids longer than C8 play a role in these processes. We also show that mtFAS- and LA-deficient strains suffer from a mild heme deficiency that may contribute to the respiratory complex assembly defect. Based on our data and previously published information, we propose a model implicating mtFAS as a sensor for mitochondrial acetyl-CoA availability and a coordinator of nuclear and mitochondrial gene expression by adapting the mitochondrial compartment to changes in the metabolic status of the cell.

Mutations in the RNA-binding protein (RBP) FUS have been genetically associated with the motoneuron disease amyotrophic lateral sclerosis (ALS). Using both human induced pluripotent stem cells and mouse models, we found that FUS-ALS causative mutations affect the activity of two relevant RBPs with important roles in neuronal RNA metabolism: HuD/ELAVL4 and FMRP. Mechanistically, mutant FUS leads to upregulation of HuD protein levels through competition with FMRP for HuD mRNA 3’UTR binding. In turn, increased HuD levels overly stabilize the transcript levels of its targets, NRN1 and GAP43. As a consequence, mutant FUS motoneurons show increased axon branching and growth upon injury, which could be rescued by dampening NRN1 levels. Since similar phenotypes have been previously described in SOD1 and TDP-43 mutant models, increased axonal growth and branching might represent broad early events in the pathogenesis of ALS.

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To better understand human health and disease, researchers create a wide variety of mouse models that carry human DNA. With recent advances in genome engineering, the targeted replacement of mouse genomic regions with orthologous human sequences has become increasingly viable, ranging from finely tuned humanisation of individual nucleotides and amino acids to the incorporation of many megabases of human DNA. Here, we examine emerging technologies for targeted genomic humanisation, we review the spectrum of existing genomically humanised mouse models and the insights such models have provided, and consider the lessons learned for designing such models in the future.