Cloning of E. coli oriC using genetic selection for ampicilin resistance and replication. Ampicilin-sensitive (Amps) E. coli were transformed with (A) a collection of circularized EcoRI fragments of E. coli DNA (but no ampicilin-resistance gene is contained on any of the fragments), with (B) a circularized EcoRI fragment containing an ampicilin-resistance gene (but this fragment does not contain an origin of replication), or with (C) a collection of circularized molecules formed by joining the EcoRI fragment containing the ampicilin-resistance gene to each of the E. coli EcoRI fragments (one of which contains oriC). (A) and (B) do not give rise to ampicilin-resistant colonies. (C) gives rise to a few ampicilin-resistant (Ampr) colonies, and each of the cells in each of these colonies contains an identical plasmid: the plasmids all have the 6.8 kb EcoRI fragment containing the ampicilin-resistance gene and a 9.0 kb EcoRI fragment containing oriC.