Genetic instability of an artificial palindrom DNA sequence.

A short DNA palindrom, produced by head to head ligation of a 29 bp DNA fragment, was inserted into a 27,000 bp plasmid DNA element composed of two functional replicons (R6K, ColE1). Several plasmid types containing a single copy of this palindrom in different locations of insertion on the R6K sequence were obtained. The palindrom was engineered to possess a unique EcoRI recognition sequence at its axis of symmetry. The presence of this restriction site allowed to monitor the genetic stability of the artificial palindrom at their different insertion loci. Out of 5 different insertion locations, one (in pAS807) was found to lead to a significant destabilization of the palindrom. This insertion site lies within the replication control region of R6K. We have shown that the inserted palindrom in pAS8O7 does not affect the functionality of the R6K replication origins. Excission of the palindrom sequences from pAS8O7 was not accompanied by loss of the adjacent R6K DNA sequences. Different deletion derivatives of pAS807 were generated in-vitro in order to determine the driving unit of DNA sequences around the palindrom that are involved in its excision. The results imply that large DNA structure(s) around the palindrom are involved in its excission. Complete deletion of R6K sequences from either the left or the right side of the palindrom resulted in new configurations which stabilized the palindrom. A configuration of R6K DNA sequences exceeding 270 bp long sequence from both sides of the palindrom are necessary for the transition from a palindrom stable to palindrom unstable state. In addition evidence is presented to show that the excision process of palindrom sequences requires a functional polymerase I but not the gene product of recA.