The incompatibility between the chromosomally integrating, conjugative transposon-like, IncJ elements R997 (ampicillin resistant) and R391 (kanamycin resistant) was examined by constructing strains harbouring both elements. Unusually, recA(+) strains harbouring the resistance determinants of both elements could be isolated but all strains lacked detectable extrachromosomal DNA. The phenotypic characteristics and transfer patterns observed suggested the formation;of recombinant hybrids lather than strains harbouring both elements independently. formation of strains harbouring two IncJ elements in a recA background was thus examined and resulted in the visualisation of extrachromosomal DNA. When R391 was transferred to a recA strain containing integrated R997, both elements co-existed stably and resulted in the isolation of a plasmid of 93.9 kb. When R997 was transferred to a recA strain harbouring an integrated R391, a plasmid of 85 kb was isolated. Comparison of restriction patterns For both elements revealed many common and several distinct fragments indicating a close physical relationship. These data suggest that although IncJ elements normally integrate at a unique site in the Escherichia coil chromosome, they possess the ability for autonomous replication which becomes manifest in a recA background when this site is occupied. This observation has implications For the nature of the incompatibility associated with IncJ elements and also provides a reliable method for isolating IncJ elements for molecular characterisation. (C) 2000 Federation of European Microbiological Societies. Published by Elsevier Science B.V, All rights reserved.