Origin and mode of replication of plasmids pE194 and pUB110

Replication of the multicopy antibiotic resistance plasmids pE194 and pUB110 has been studied in Bacillus subtilis. Based on electron microscopic analysis of replication intermediates and on evidence from deletants, we have determined that both plasmids replicate unidirectionally from fixed origins by a Θ mechanism. The location of these origins and the directions of replication have been determined relative to the physical maps. A cointegrate of pE194 and pUB110 has also been studied and evidence is presented from electron microscopy, temperature resistance, and copy number analysis that the cointegrate uses both parental replication origins.