Monocyte chemotactic protein-1 secretion and expression after Toxoplasma gondii infection in vitro depend on the stage of the parasite

We constructed a series of plasmids that allow the insertion of cloned DNA in the Escherichia coli chromosome by site-specific integration into the bacteriophage HK022 bacterial attachment site. These plasmids make use of a ColE1 origin of replication, the phage HK022 attachment site attP, antibiotic resistance genes for selection and unique restriction sites. Circularisation of non-replicative fragments containing the HK022 attachment site attP is performed in vitro and site-specific integration of attP containing molecules is ensured by transfer into cells transiently expressing the HK022 integrase gene carried by a thermosensitive replicon. Insertion is very efficient and the inserted fragments are stably maintained without selection pressure. Since integrative fragments carry rarely used antibiotic markers conferring resistance to antibiotics hygromycin or apramycin, they can be used in most E. coli strains in conjunction with many replicative or integrative vectors.