Abstract
The R1 origin region contains many symmetrical DNA sequence elements which allow the formation of complex secondary structures. A 218-bp in vivo deletion in a cloned R1 origin fragment removes most of them. As this deletion was never observed in plasmids containing all R1 replication functions, it was introduced by BglI in vitro recombination into the ;basic replicon' of R1 cloned into pBR322. The recombinant plasmid with the 218-bp deletion and its derivatives unambiguously show that the deleted symmetrical elements are not absolutely essential for R1 replication as was previously assumed though they seem to determine a more efficient origin function. Likewise, a hypothetical protein of a mol. wt. of 14 000 daltons, the major part of which would be encoded by the deleted sequences, does not seem to be of particular importance for R1-specific replication. This is the first report of an alteration in the origin region of an IncFII plasmid which affects plasmid replication without abolishing it completely.