Heterogeneity among the 2 microns plasmids in Saccharomyces cerevisiae: a new sequence for the REP1 gene

Some species of yeasts contain naturally-occurring circular DNA plasmids. The most studied of these plasmids is the 2 microns circle of Saccharomyces cerevisiae. Three variants of this plasmid, Scp1, Scp2 and Scp3, have been described according to their restriction maps [Cameron et al., Nucleic Acids Res. 4 (1977) 1429-1448; Livingston, Genetics 86 (1977) 73-84]. The entire nucleotide (nt) sequence of the Scp1 variant from strain A364A has been published [Hartley and Donelson, Nature 286 (1980) 860-864]. We report here the nt sequence of the 2 microns plasmid REP1 gene from S. cerevisiae strain SKQ2n. According to the restriction analysis, this plasmid is the Scp3 variant previously described. The only observed differences between the Scp1 and Scp3 variants were the loss of one EcoRI restriction site and an apparent deletion in Scp3. The nt sequence we report differs significantly from the previously published one for Scp1. The differences correspond to 128 (about 8.5%) substituted, deleted or additional nt of 1510 nt compared. These differences affect the coding region (8%) as well as the noncoding regions (9.7%). Regarding the putative encoded proteins, 38 (about 10%) amino acids (aa) are modified or deleted in our sequence and 11 are added. Most of these aa modifications are not randomly distributed but are concentrated in certain regions. These observations are indicative of important intraspecific evolution between the two 2 microns plasmid variants considered, as well as of conservative selection pressure on some domains of the REP1 protein.