Fig. 3.

Illustration of important processes in plasmid biology with examples of three different plasmids. (a) The low-copy-number (~1 copy per cell) plasmids p42a (conjugative) and p42d (nontransmissible) of Rhizobium etli CFN42 (GenBank accession numbers CP000134.1 and U80928.5 [256, 257]) fuse into a cointegrate and then are transferred by conjugation from a host to a new recipient. The two cells have been pulled together by a pilus, which has then been retracted. A single strand of a plasmid copy is transferred, while the other strand remains within the cell in circular form. Afterward, a second strand will be synthesized in both cells to generate double-stranded DNA. p42d is also referred to as pSym, since it contains most of the genes responsible for rhizobial symbiosis in this strain; it is normally transferred horizontally by cointegration with p42a [191, 258]. (b) The copies of the small, high-copy-number (~14 copies per cell) resistance plasmid pB1000 (GenBank accession number GU080070.1 [180]) have been segregated between daughter cells at host division; cell division is almost complete. Inside each bacterial cell, the plasmids and chromosome are depicted. The diagrams of the plasmids show the open reading frame* of each gene on the plasmid as an arrow coloured by the function of the encoded protein. The region of pB1000 marked in the replication colour contains the plasmid origin of replication and the coding sequences for the RNAs involved in regulation of replication. The origins of replication of p42a and p42d are located inside the RepA gene; the origin of transfer of p42a is marked with a dot in the mobilization colour.