EPISTATIC EFFECTS OF PROMOTER AND REPRESSOR FUNCTIONS OF THE TN10 TETRACYCLINE-RESISTANCE OPERON ON THE FITNESS OF ESCHERICHIA-COLI

We have been studying the effects of expression of plasmid-borne, Tn10 -encoded, tetracycline resistance on the fitness of Escherichia coli K 12. We previously demonstrated large reductions in fitness resulting f rom induced or constitutive expression of the resistance protein; howe ver, any residual expression by the repressed operon was so slight tha t possession of an inducible resistance function imposed essentially n o burden in the absence of antibiotic. Here, we demonstrate two distin ct disadvantages for inducible genotypes relative to isogenic constitu tive constructs. During the transition from antibiotic-free to antibio tic-containing media, the inducible genotype experiences a longer lag phase prior to growth. In the sustained presence of antibiotic, full i nduction of the resistance function in the inducible genotype is preve nted by the continued action of its repressor. However, these disadvan tages may be reduced by increasing the strength of the promoter for th e resistance gene in the inducible genotype. Simultaneous consideratio n of the mode of gene regulation (i.e. constitutive or inducible) and the strength of the resistance-gene promoter (i.e. maximum level of ex pression) indicates an adaptive landscape with very strong epistasis a nd, perhaps, multiple fitness peaks.