TRANSCRIPTION INCREASES THE DELETION FREQUENCY OF LONG CTG-CENTER-DOT-CAG TRIPLET REPEATS FROM PLASMIDS IN ESCHERICHIA-COLI

Induction of transcription into long CTG.CAG repeats contained on plas mids in Escherichia coli is shown to increase the frequency of deletio ns within the repeat sequences, This elevated genetic instability was detected because active transcription into the triplet repeat influenc ed the growth transitions of the host cell, allowing advantageous grow th for cells harboring plasmids with deleted repeat sequences, The var iety of deletion products observed in separate cultures suggests that transcription altered the metabolism of the DNA in a manner that produ ced random length changes in the repeat sequence, For cultures contain ing plasmids without active transcription into the triplet repeat, or those maintained in exponential growth, deletions occurred within the repeat at a lower frequency (5-20-fold lower), In these incubations th e extent of deletions was proportional to the number of cell divisions and many repeat lengths were observed within each culture, suggesting that the decrease in average repeat length at long incubation times w as due to multiple small deletions, These observations show that delet ions within long CTG.CAG repeats contained on plasmids in E. coli occu r via more than one pathway and their level of genetic instability is altered by the enzymatic processes occurring upon the DNA.