Type I Restriction and Modification Systems

The R124 Plasmid

DNA rearrangements of R124

The R124 Plasmid (EcoRI restriction map)

The R124 plasmid encodes the first described member of the Type IC family - EcoR124I and was the first F-like plasmid found to encode a Restriction-Modification system. However, the history of R124 is both confused and littered with errors. Therefore, in and attempt to clarify some of these confusions a recent review of the history of work with the EcoR124I R-M system was written by James Youell and Keith Firman.

The IncFIV plasmid R124 is a 120 kbp conjugative plasmid that carries the hsd genes of the EcoR124I R-M system and tetracycline resistance (on the transposon Tn10). However, it can be argued that the classification as an IncFIV plasmid, based on the observed co-existence with a wide range of plasmids, is somewhat suspect because the observed DNA rearrangements that the plasmid undergoes in the presence of the IncFI plasmid F⁺ (Firman et al 1983; Glover et al 1983). What is particularly interesting about these rearrangements is the involvement of regions of both plasmids that are associated with DNA replication - suggesting DNA rearrangements may overcome some incompatibility issues. In addition, these rearrangements involve the hsd genes, resulting in deletion or rearrangement that also produced DNA specificity changes within the hsdS gene. However, the precise nature and mechanism for these rearrangements remains unknown, but suggest that the full DNA sequence of both R124 and the rearranged plasmids is required before this situation can be fully understood.

Some of the errors or misunderstandings associated with R124 and the EcoR124I R-M system are:

The plasmid was originally isolated from Salmonella typhimurium and one could argue the enzyme should be called StyR124I.
As mentioned above the plasmid was classified as an IncFIV, but DNA rearrangements occur in the presence of the supposedly compatible F-plasmid, which suggests that the two plasmids may NOT be compatible and that R124 Is perhaps an IncFI plasmid.
The EcoR124I R-M system was at one time mistaken for the newly discovered EcoRI system - this is NOT the case and EcoR124I is a Type I R-M system, while EcoRI is a Type 2 R-M system.
The EcoR124II R-M system was isolated from a population of R124 plasmids that had bben transferred to a xth mutant strain of E. coli as one of four ex-conjugants obtained, there is no clear explanation for why this change in DNA specificity occurred.