Generating a Small Shuttle Vector for Effective Genetic Engineering of Methanosarcina mazei Allowed First Insights in Plasmid Replication Mechanism in the Methanoarchaeon

Due to their role in methane production, methanoarchaea are of high ecological relevance and genetic systems have been ever more established in the last two decades. The system for protein expression in Methanosarcina using a comprehensive shuttle vector is established; however, details about its replication mechanism in methanoarchaea remain unknown. Here, we report on a significant optimisation of the rather large shuttle vector pWM321 (8.9 kbp) generated by Metcalf through a decrease in its size by about 35% by means of the deletion of several non-coding regions and the ssrA gene. The resulting plasmid (pRS1595) still stably replicates in M. mazei and—most likely due to its reduced size—shows a significantly higher transformation efficiency compared to pWM321. In addition, we investigate the essential gene repA, coding for a rep type protein. RepA was heterologously expressed in Escherichia coli, purified and characterised, demonstrating the significant binding and nicking activity of supercoiled plasmid DNA. Based on our findings we propose that the optimised shuttle vector replicates via a rolling circle mechanism with RepA as the initial replication protein in Methanosarcina. On the basis of bioinformatic comparisons, we propose the presence and location of a double-strand and a single-strand origin, which need to be further verified.