Variations in expression of the genes of the NAH7 (naphthalene biodegradation) plasmid of when placed in different chromosomal locations in have been studied by employing a collection of hybrid mini-Tn transposons bearing fusions to the promoter, along with the cognate regulatory gene Insertions of reporters in the proximity of the chromosomal origin of replication, increased accumulation of β-galactosidase 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 elements. Such variations in β-galactosidase activity (and, therefore, the expression of catabolic genes) seemed, instead, to be linked to the increase in gene dosage associated with regions close to and not to local variations in chromosome structure. The tolerance of strains to the selection markers borne by the transposons also varied in parallel with the changes in LacZ levels. The role of chromosomal positioning as a mechanism for the outcome of adaptation phenotypes is discussed.


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