The story of Rhodobacter capsulatus gene transfer agent: Virus, organelle or both?

P. Bárdy^1,2, T. Füzik², D. Hrebík², R. Pantůček¹, J.T. Beatty³, P. Plevka²

¹Department of Experimental Biology, Masaryk University, Kamenice 5, 62500 Brno, Czech Republic

²Central European Institute of Technology, Masaryk University, Kamenice 5, 62500 Brno, Czech Republic

³Department of Microbiology and Immunology, University of British Columbia, 2350 Health Sciences Mall, Vancouver, V6T 1Z3, B.C., Canada

bardy.pavol@mail.muni.cz

Gene transfer agents (GTAs) are DNA-containing bacteriophage-like particles produced by alphaproteobacteria, the most abundant microorganisms of temperate oceans. Here, we present the structure and genome-delivery mechanism of Rhodobacter capsulatus GTA (RcGTA) solved by cryo- EM. RcGTA differs from phages by presence of oblate capsid shortened in the direction of the tail axis which limits its infectious potential. The capsid is decorated with receptor-binding proteins responsible for the initial attachment of RcGTA to a recipient capsule. Tail of RcGTA possess features of both long a short-tailed phages, resembling their common ancestor. DNA-ejection is triggered by conformation change of the baseplate, resulting in exposure of membrane-penetrating helices. Interaction of helices with the membrane triggers release of cell-wall peptidase, tape-measure protein and DNA into the recipient periplasm, from where the DNA is delivered to cytoplasm by bacterial transformation-like complex. This study identifies key structural features of nanodelivery vehicles and helps us understand gene transmission processes in ocean environments.

Figure 1. Cryo-EM reconstruction of a native particle of RcGTA. The left part of the figure shows the complete particle, whereas on the central the front half of the particle has been removed to show DNA and internal proteins. On the right the T=3 isometric variant of RcGTA, representing one percent of the observed particles, is shown.

CIISB research infrastructure project – LM2015043, IT4Innovations National Supercomputing Center – LM2015070, and the National Grid Infrastructure MetaCentrum  – LM2015042 funded by by The Ministry of Education, Youth and Sports of the Czech Republic are gratefully acknowledged. This work was supported by Masaryk University funding MUNI/E/0530/2019, MUNI/A/1127/2019 and Czech Science Foundation grants 15-21631Y, 18-17810S and 18-13064S.