Buss, J. | Search bioRxiv

bioRxiv2018

An activation pathway governs cell wall polymerization by a bacterial morphogenic machine

Cell elongation in rod-shaped bacteria is mediated by the Rod system, a conserved morphogenic complex that spatially controls cell wall (CW) assembly. In Escherichia coli, alterations in a CW synthase component of the system called PBP2 were identified that overcome other inactivating defects. Rod system activity was stimulated in the suppressors in vivo, and purified synthase complexes with these changes showed more robust CW synthesis in vitro. Polymerization of the actin-like MreB component of the Rod system was also found to be enhanced in cells with the activated synthase. The results suggest an activation pathway governing Rod system function in which PBP2 conformation plays a central role in stimulating both CW glycan polymerization by its partner RodA and the formation of cytoskeletal filaments of MreB to orient CW assembly. An analogous activation pathway involving similar enzymatic components is likely responsible for controlling CW synthesis by the division machinery.

microbiology↗

bioRxiv2018

Asymmetric histone incorporation during DNA replication in Drosophila male germline stem cells

One Sentence SummaryDNA replication establishes asymmetric epigenomes SummaryOne of the most fundamental questions in developmental biology concerns how cells with identical genomes differentiate into distinct cell types. One important context for understanding cell fate specification is asymmetric cell division, where the two daughter cells establish different cell fates following a single division. Many stem cells undergo asymmetric division to produce both a self-renewing stem cell and a differentiating daughter cell1-5. Here we show that histone H4 is inherited asymmetrically in asymmetrically dividing Drosophila male germline stem cells, similar to H36. In contrast, both H2A and H2B are inherited symmetrically. By combining superresolution microscopy with the chromatin fiber method, we are able to study histone inheritance patterns on newly replicated chromatin fibers. Using this technique, we find asymmetric inheritance patterns for old and new H3, but symmetric inheritance patterns for old and new H2A on replicating sister chromatids. Furthermore, co-localization studies on isolated chromatin fibers and proximity ligation assays on intact nuclei reveal that old H3 are preferentially incorporated by the leading strand while newly synthesized H3 are enriched on the lagging strand. Finally, using a sequential nucleoside analog incorporation assay, we detect a high incidence of unidirectional DNA replication on germline-derived chromatin fibers and DNA fibers. The unidirectional fork movement coupled with the strand preference of histone incorporation could explain how old and new H3 are asymmetrically incorporated by replicating sister chromatids. In summary, our work demonstrates that the intrinsic asymmetries in DNA replication may help construct sister chromatids enriched with distinct populations of histones. Therefore, these results suggest unappreciated roles for DNA replication in asymmetrically dividing cells in multicellular organisms.

developmental biology↗