Lactose operon genes

Fig. 24.5 Insertion of a cloned insulin gene into a vector carrying a bacterial promoter. The arrow indicates the direction of transcription. If we suppose the bacterial promoter is derived from the lactose operon then transcription will be initiated only in the presence of lactose.

Answer E. TBP and CBP participate in eukaryotic gene transcription and mRNA translation, respectively. CAP regulates the expression of prokaryotic lactose operons. ACP is involved in fatty add synthesis. 

Poll expression of the lactose operon requires that both mechanisms favor gene expression. 

The well-investigated lactose operon of the bacterium Escherichial coli can be used here as an example of transcriptional control. The lac operon is a DNA sequence that is simultaneously subject to negative and positive control. The operon contains the structural genes for three proteins that are required for the utilization of lactose (one transporter and two enzymes), as well as control elements that serve to regulate the operon. 

A variety of different promoters have been used for the expression of antibody genes. Widely used is the lacZ promoter (lacZ) derived from the lactose operon (53). The gill promoter (gill) from the bacteriophage M13 (9), the tetracycline promoter (IX teto/p ref. 54) and the phoA promotor of the E. coli alkaline phosphatase (47) also have been used successfully. It appears that very strong promoters, for example, the synthetic promoter PAI/04/03 (55), are... 

The end products of gene expression are proteins, mainly enzymes, and it is essential that their levels be strictly controlled. There are many potential sites of control in both bacteria and eukaryotes. DNA or gene amplification (Chap. 16) in eukaryotes is one way of responding to the demand for more of the protein product if there arc more copies of the gene, then transcription can occur at a faster rate. More often, control is effected at the level of cither transcription or translation, with the former probably being more important for both bacteria and eukaryotes. Transcriptional control in bacteria is particularly effective because of the very short half-life (a few minutes) of mRNA in such cells the half-life is longer in eukaryotes. The prototype for transcriptional control is the lactose operon in E. coli. 

Fig. 17-13 The lactose operon of E. coli. Abbreviations i = repressor gene ...

The regulation of bacterial transcription is well illustrated by the lactose operon (lac operon) of the colon bacterium Escherichia coli in which the upstream region successively (from the 5 end of the sense strand) includes a promoter (P ) for the gene (I) coding for a repressor protein (the lac repressor), a CRP binding site , the promoter for the lac operon (P), and finally an operator site (O) that prefaces the Z, Y and A structural genes of the operon ... 

Figure 31.3. Operons. (A) The general structure of an operon as conceived by Jacob and Monod. (B) The structure of the lactose operon. In addition to the promoter (p) in the operon, a second promoter is present in front of the regulator gene (/) to drive the synthesis of the regulator.

Operon composition/induction - The lactose operon consists of three linked structural genes that encode enzymes of lactose utilization, plus adjacent regulatory sites. The three structural genes—z, y, and a—encode / -galactosidase, / -galactoside permease (a transport protein), and thiogalactoside transacetylase (an enzyme of still unknown metabolic function), respectively. 

The gene products of the lactose operon (see here) are not needed unless lactose is also present to be consumed. A different situation is encountered with genes whose products catalyze biosynthesis. 

The Lactose Operon Early Evidence for Transcriptional Control of Gene Expression... 

Some of the known constitutive mutations of the lactose operon occur in the operator sequence rather than the regulator gene. 

EXAMPLE 9.38 The lactose operon (or lac operon) is a region of 5.3 kb of the E. coli chromosome that contains the genes that encode the three enzymes that catabohze lactose. The cell stringently controls expression of the lac genes it expresses them only if it needs to metabolize lactose. Fnrthermore, when glncose is also present, there is no need for high levels of expression of the lac operon. Therefore, as detailed in Fig. 9-17, the lac operon is nnder both negative... 

Fig. 9-17 The lactose operon of E. coll. Here I, p, o, z, y, and a denote the repressor gene, promoter, operator, p-galactosidase gene, permease gene, and transacetylase genes, respectively. Because the three genes, z, y and a, are transcribed as a single unit polycistronic mRNA), they are said to be expressed coordinately. When transcription is blocked by the binding of the repressor to the operator, none of the genes are expressed.

Nissen, L., Perez-Martinez, G., and Yebra, M.J. (2005) Sorbitol synthesis by an engineered Lactobacillus casei strain expressing a sorbitol-6-phosphate dehydrogenase gene within the lactose operon. FEMS Microbiol Lett, 249, 177-183. 

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