Conjugal Plasmids and Their Transfer

Abstract

Virtually all members of the family Rhizobiaceae that have been examined contain one or more plasmids. Most of these elements are large and some are known to play key roles in the biology of these bacteria, especially with respect to the association between these microorganisms and their plant hosts. Structure-function studies indicate that, in a given species, a particular plasmid specific to that isolate may contain regions that are conserved among plasmids present in other isolates of the same species, in isolates of other species of that genus, or even in isolates of other genera of the family. This suggests several important concepts that tie together the various species and genera of the family Rhizobiaceae. First, that these plasmids often share substantial regions of similar sequence suggests that the individual elements have arisen by recombination with other plasmids. However, these plasmids often exhibit equally large regions of non-similarity indicating that they maintain diversity. Thus, some sym plasmids share substantial sequence relatedness with some Ti plasmids. But these two plasmid types also contain large regions of unrelated sequence. This, of course is what defines one set as sym plasmids and the other as Ti plasmids. However, that they share regions of relatedness suggests that some core portion of these plasmids may be common and indeed essential for the maintenance of these elements as well as for the recombination events that lead to their diversity. This suggests, in turn, that these plasmids are capable of transferring between bacteria of the same family and perhaps bacteria of more distantly related taxa. Recent analyses indicate that the replication regions of many Rhizobium plasmids are related to those of several Ti and Ri plasmids (Turner and Young, 1995; Turner, et al., 1996) and that these plasmids may all fall into a superfamily of extrachromosomal elements sharing a common core replicator region. More intriguingly, as will be discussed below, this conserved replicator region is intimately linked to a bank of conserved genes associated with conjugal transfer in at least two Rhizobium plasmids (Turner, et al., 1996; Freiberg, et al., 1997), as well as in several Ti plasmids (Alt-Mörbe, et al., 1997; Li and Farrand, unpublished). Thus, a common replication machinery, linked with a conjugal transfer system, may form the core structure of many of the important plasmids present in the various members of the family Rhizobiaceae, Furthermore, available sequence data suggests that these large plasmids take on and evolve their characters by recombination with other plasmids or components of the bacterial genome. Thus, the conjugal transfer systems may be of particular importance to the evolution, as well as the dissemination of these plasmids. The nature of the conjugal transfer systems of these plasmids and the significance of conjugation with respect to the variability found within the plasmids of the members of this family forms the nucleus of this review.