MODULATION OF GENE-EXPRESSION THROUGH CHROMOSOMAL POSITIONING IN ESCHERICHIA-COLI

Variations in expression of the nah genes of the NAH7 (naphthalene bio degradation) plasmid of Pseudomonas putida when placed in different ch romosomal locations in Escherichia coil have been studied by employing a collection of hybrid mini-Tn5 transposons bearing lacZ fusions to t he Psal promoter, along with the cognate regulatory gene nahR. Inserti ons of Psal-lacZ reporters in the proximity of the chromosomal origin of replication, oriC, increased accumulation of beta-galactosidase in vivo. Position-dependent changes in expression of the reporter product could not be associated with local variations of the supercoiling in the DNA region, as revealed by probing the chromosome with mobile gyrB -lacZ elements. Such variations in beta-galactosidase activity (and, t herefore, the expression of catabolic genes) seemed, instead, to be li nked to the increase in gene dosage associated with regions close to o riC, and not to local variations in chromosome structure. The toleranc e of strains to the selection markers borne by the transposons also va ried in parallel with the changes in LacZ levels. The role of chromoso mal positioning as a mechanism for the outcome of adaptation phenotype s is discussed.