Design of Bacterial Hosts for lac-Based Expression Vectors

Since introduction of the first pUC plasmids (1 ), a great variety of plasmid vectors that use α-complementation and expression from the lac promoter, or its derivatives tac and trc promoters, have been developed (e.g., see refs. 2 –14 ). In order to maximize utilization of these vectors, various Escherichia coli host strains have been designed which contain the lacZ ΔM15 allele (15 ) necessary for α-complementation and the lacI ^q (16 ,17 ) gene, which allows for overproduction of the lac repressor that is required for regulated expression from the lac promoter. The development of F episomes (7 ,14 ,18 ) or phages (1 ,19 ) containing these components facilitated the construction of various host strains, provided they do not express β-galactosidase (e.g., Δlac strains). However, these systems suffer from several short-comings that restrict their use:

1.	Unless the episomes contain transposon-encoded antibiotic resistance markers (usually kanamycin or tetracycline), which also excludes their use in Tn5 - or Tn10 -containing strains, other commonly used F episomes require minimal medium for their maintenance because passage in rich media leads to their frequent loss (20 ),
2.	Since the episomes and phages have been tailored for use in E. coli , they cannot be exploited for establishment of a lac -based α-complementation and regulated expression system in other bacteria.