Topological modifications and template activation are induced in chimaeric plasmids by inserted sequences.

Abstract

The effect of the insertion of foreign genes or gene systems in closed DNA domains has been investigated in vitro in purified systems. We observe that in chimaeric plasmids two apparently independent classes of modifications, (1) functional and (2) topological, do take place in defined instances. (1) Among the screened yeast gene systems, examples have been found of DNA sequences that upon insertion cause activation of in vitro transcription of distant genes. (2) Foreign DNA sequences may lead to new topological features of the harbouring plasmids; it is shown that more than one S1-sensitive secondary structure may be contemporaneously present on the same chimaeric plasmid. DNA superhelicity is a prerequisite of these modifications. The two classes of effects (1) functional and (2) topological are not a priori directly related one to the other but appear to be two independent consequences of the same cause: the insertion of foreign DNA sequences into closed DNA domains. These observations suggest a regulatory model of gene expression based on alternative topologies of closed DNA domains.