Insertion into and release of the cytoplasmic domain of the Schizosaccharomyces pombe spindle pole body from a nuclear envelope fenestra during mitosis requires Brr6.

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During translation, the two eukaryotic ribosomal subunits remain associated through 17 intersubunit bridges, five of which are eukaryote-specific. These are mainly localized to the peripheral regions and are believed to stabilise the structure of the ribosome. The functional importance of these bridges remains largely unknown. Here the essentiality of the eukaryote-specific bridge eB12 has been investigated. The main component of this bridge is ribosomal protein eL19 which is composed of an N-terminal globular domain, a middle region and a long C-terminal α-helix. Analysis of deletion mutants demonstrated that the globular domain and middle region of eL19 are essential for cell viability, most likely functioning in ribosome assembly. The eB12 bridge, formed by contacts between the C-terminal α-helix of eL19 and 18S rRNA in concert with additional stabilising interactions involving either eS7 or uS17, is dispensable for viability. Nevertheless, eL19 mutants impaired in eB12 bridge formation displayed slow growth phenotypes, altered sensitivity/resistance to translational inhibitors and enhanced hyperosmotic stress tolerance. Biochemical analyses determined that the eB12 bridge contributes to the stability of ribosome subunit interactions in vitro. 60S subunits containing eL19 variants defective in eB12 bridge formation failed to form 80S ribosomes regardless of Mg2+ concentration. The reassociation of 40S and mutant 60S subunits was markedly improved in the presence of deacetylated tRNA, emphasising the importance of tRNAs during the subunit association. We propose that the eB12 bridge plays an important role in subunit joining and in optimizing ribosome functionality.

Saccharomyces cerevisiae mitochondrial DNA polymerase (Mip1) contains a C-terminal extension (CTE) of 279 amino acid residues. The CTE is required for mitochondrial DNA maintenance in yeast but is absent in higher eukaryotes. Here we use recombinant Mip1 C-terminal deletion mutants to investigate functional importance of the CTE. We show that partial removal of the CTE in Mip1Δ216 results in strong preference for exonucleolytic degradation rather than DNA polymerization. This disbalance in exonuclease and polymerase activities is prominent at suboptimal dNTP concentrations and in the absence of correctly pairing nucleotide. Mip1Δ216 also displays reduced ability to synthesize DNA through double-stranded regions. Full removal of the CTE in Mip1Δ279 results in complete loss of Mip1 polymerase activity, however the mutant retains its exonuclease activity. These results allow us to propose that CTE functions as a part of Mip1 polymerase domain that stabilizes the substrate primer end at the polymerase active site, and is therefore required for efficient mitochondrial DNA replication in vivo.