Operons constitutively expressed

In addition, the i gene, which encodes the lac repressor protein, is also considered part of the operon although it is located at a distant site in the DNA. The i gene is constitutively expressed (not regulated) thtis, copies of the lac repressor protein are always in the cell. 

HuX, Mamoto R, Fujioka Y, Tani A, Kimbara K, Kawai F (2008) The pva operon is located on the megaplasmid of Sphingopyxis sp. strain 113P3 and is constitutively expressed, although expression is enhanced by PVA. Appl Microbiol Biotechnol 78 685-893... 

The evidence that we have presented indicates that the allele (in the absence of L-arabinose as the inducer) does produce two functionally different products the activator and the repressor. The activator function is detected by the stimulatory effect of the aUele on the ara operon cis to deletion 719 and to certain r mutants in strains containing deletion 719. The repressor function is indicated by the epistatic effect of the allele on the constitutive expression of the ara operon cis to I" A766 and by the dominance of to O as indicated previously. That one function is transformed into another is evidenced by the inducibility of the pertinent merodiploid invoked by L-arabinose. Thus it appears that L-arabinose removes the repressor from the operator site and shifts the equilibrium to the activator, which then reacts at aral and stimulates the expression of the operon (Fig. 3). 

The vector is used to express the genes of the sox operon (soxA, soxB, and soxC) from the constitutive promoter and so, Rhodococcus, Gordona, and Nocardia may be transformed with this vector and used in BDS of fossil fuels. Equivalent patents GB0311395D, ITMI20021217. 

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