The origin of replication region contains segments of RNAI and RNAII and the segments are of 108 and 465 nucleotides long, respectively. Both RNA I and RNA II are found to involve in replication of the plasmid. In pRK10, transcription of RNA Istarts at position 1387 and proceeds in the direction opposite to the RNA II transcription and terminates near the site where transcription of RNA IIstarts. Thus, RNA Iis complementary to the 5’ portion of RNA II.In ColEl, 5’ portion of RNA IIis involved in regulation of plasmid DNA replication. RNA Ibinds to RNA II( Tomizawa 1984; Tomizawa and Itoh, 1981 ) and the binding prevents formation of a secondary structure of RNA IIthat is required for the transcript to form the primer for DNA synthesis by cleavage with RNAse H. In the case of ColE1, the rop gene is partially responsible for maintaining the plasmid copy number (Cesareni et al., 1982). On the other hand, similar to ColA (Morlon et al., 1988), pRK10 does not have a gene equivalent to rop for copy number control and the 553-bp ori region or maybe the RNAI itself is probably sufficient for maintaining the plasmid copy number. The corresponding -10 and -35 sequences of RNAI and RNAII of pRK10 are highly conserved among ColE1, p15A, and pSW200.

The bom(basis of mobility) region is a cis-requiring region for plasmid mobility (Finnegan and Sherratt, 1982). Plasmid pRK10 also contains sequence homologous to the bom region of ColE1. The homology of the bom regions between the two plasmids pRK10 and ColE1 is 70.7%. This region is located between nt 1854 and nt 2147 in pRK10. A putative oriT nick site in the bom region of pRK10 is at 2024. The structural genes encoding Colicin E1 or Colicin E1 immunity protein were not found on the plasmid pRK10