RepB C-terminus mutation of an ori pRi vector affects plasmid copy number in Agrobacterium and transgene copy number in plants

A native replication origin, ori pRi, was previously reported as a single copy plasmid in and can improve the production of transgenic plants with a single copy insertion of transgenes when it is used in binary vectors for transformation. A high copy ori pRi variant plasmid, pTF::Ri, which does not improve the frequency of single copy transgenic plants, has been reported in the literature. Sequencing the high copy pTF::Ri operon revealed the presence of two mutations: one silent mutation and one missense mutation that changes a tyrosine to a histidine (Y299H) in a highly conserved area of the C-terminus of the RepB protein (RepB). Reproducing these mutations in the wild-type oriRi binary vector showed that cells with the RepB mutation grow faster on both solidified and in liquid medium, and have higher plasmid copy number as determined by ddPCR. In order to investigate the impact of the RepB mutation on transformation and quality plant production, the RepB mutated ori pRi binary vector was compared with the original wild-type ori pRi binary vector and a multi-copy oriV binary vector in canola transformation. Molecular analyses of the canola transgenic plants demonstrated that the multi-copy ori pRi with the RepB mutation in cells lost the advantage of generating high frequency single copy, backbone-free transgenic plants compared to using the single copy wild-type ori pRi binary vector.