pING family of conjugative plasmids from the extremely thermophilic archaeon Sulfolobus islandicus: Insights into recombination and conjugation in Crenarchaeota

A novel family of conjugative plasmids from Sulfolobus comprising the activ e variants pING1, -4, and -6 and the functionally defective variants pING2 and -3, which require the help of an active variant for spreading, has been extensively characterized both functionally and molecularly, In view of th e sparse similarity between bacterial and archaeal conjugation and the lack of a practical genetic system for Sulfolobus, we compared the functions an d sequences of these variants and the previously described archaeal conjuga tive plasmid pNOB8 in order to identify open reading frames (ORFs) and DNA sequences that are involved in conjugative transfer and maintenance of thes e plasmids in Sulfolobus. The variants pING4 and -6 are reproducibly derive d from pING1 in vivo by successive transpositions of an element from the Su lfolobus genome. The small defective but mobile variants pING2 and -3, whic h both lack a cluster of highly conserved ORFs probably involved in plasmid transfer, were shown to be formed in vivo by recombinative deletion of the larger part of the genomes of pING4 and pING6, respectively. The efficient occurrence of these recombination processes is further evidence for the st riking plasticity of the Sulfolobus genome.