Choi Wonyoung, Saha Rudra P, Jang Sooin, Harshey Rasika M
Department of Molecular Biosciences & Institute of Cellular and Molecular Biology, University of Texas at Austin, Austin, TX, 78712, USA.
Mol Microbiol. 2014 Nov;94(3):595-608. doi: 10.1111/mmi.12781. Epub 2014 Sep 25.
Phage Mu is unique among transposable elements in employing a transposition enhancer. The enhancer DNA segment is the site where the transposase MuA binds and makes bridging interactions with the two Mu ends, interwrapping the ends with the enhancer in a complex topology essential for assembling a catalytically active transpososome. The enhancer is also the site at which regulatory proteins control divergent transcription of genes that determine the phage lysis-lysogeny decision. Here we report a third function for the enhancer - that of regulating degradation of extraneous DNA attached to both ends of infecting Mu. This DNA is protected from nucleases by a phage protein until Mu integrates into the host chromosome, after which it is rapidly degraded. We find that leftward transcription at the enhancer, expected to disrupt its topology within the transpososome, blocks degradation of this DNA. Disruption of the enhancer would lead to the loss or dislocation of two non-catalytic MuA subunits positioned in the transpososome by the enhancer. We provide several lines of support for this inference, and conclude that these subunits are important for activating degradation of the flanking DNA. This work also reveals a role for enhancer topology in phage development.
噬菌体Mu在转座元件中独一无二，它利用一个转座增强子。增强子DNA片段是转座酶MuA结合的位点，并与两个Mu末端进行桥接相互作用，以一种复杂的拓扑结构将末端与增强子缠绕在一起，这种拓扑结构对于组装具有催化活性的转座体至关重要。增强子也是调节蛋白控制决定噬菌体裂解-溶原决定的基因的发散转录的位点。在这里，我们报道了增强子的第三种功能——调节附着在感染性Mu两端的外来DNA的降解。在Mu整合到宿主染色体之前，这种DNA受到噬菌体蛋白的保护而免受核酸酶的作用，之后它会迅速降解。我们发现，增强子处的向左转录，预计会破坏其在转座体内的拓扑结构，从而阻止这种DNA的降解。增强子的破坏将导致位于转座体中的两个非催化性MuA亚基因增强子而丢失或错位。我们为这一推断提供了几条证据，并得出结论，这些亚基对于激活侧翼DNA的降解很重要。这项工作还揭示了增强子拓扑结构在噬菌体发育中的作用。
