Ubiquinone (Coenzyme Q) is an essential component of bacterial respiratory chains. The first committed step in the biosynthetic pathway is the formation of 4-hydroxybenzoate from chorismate by the enzyme chorismate pyruvate-lyase encoded by the uhiC gene. The 4-hydroxyhenzoate is prenylated by 4-hydroxybenzoate octaprenyltransferase encoded by the ubiA gene, The two genes are linked at L, 91.5 min in the Escherichia coli chromosome. To study the regulation, operon fusions were constructed between these two genes and the lacZ gene. The fusions were introduced into the chromosome as a single copy at the lambda attachment site. Expression of P-galactosidase was determined in strains carrying the operon fusions ubiC-lacZ' ubiCA'-IacZ', and ubiA'-IacZ'. In glycerol media, the highest level of expression was observed with the operon fusion ubiC'-lacZ(+). Compared with the ubiC-lacZ(+), the ubiCA'-IacZ(+) operon fusion showed 26% of the activity while the ubiA'-lacZ(+) operon fusion had an activity of 1%. Thus, the ubiC gene is regulated by the upstream promoter while the ubiA gene lacks its own promoter. The effect of fermentable and oxidizable carbon sources on the expression of ubiC'-lacZ(+) was determined. The expression was low in the case of a fermentable carbon source, glucose, while in the presence of oxidizable carbon sources the expression increased 2- to 3-fold. When the expression of ubiC-lacZ(+) and ubiCA'-IacZ(+) operon fusions were compared under a wide variety of conditions, the levels of P-galactosidase varied coordinately, suggesting that the ubiCA genes are organized into an operon. The variations in transcription of the operon under different nutritional conditions and in the regulatory mutants, arcA,fnr, and narXL are presented.