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I was reading BIOS Biochemistry and stumbled upon a phrase 'background level' in the following excerpt $-$

The lac operon contain lacZ,lacY and lacA genes encoding $beta$-galactosidase, galactoside permease and thiogalactoside transacetylase, respectively, preceded by an operator site and a promoter. The operon is transcribed by RNA polymerase to produce a single polycistronic mRNA that is translated to produce all three enzymes. When lactose is present the background level of $beta$-galactosidase converts some lactose to allolactose, which induces transcription of the lac operon.

What is meant by background level? Does it refer to the basal level of transcription, the constitutive transcription of facultative genes?

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    $\begingroup$ Why do you see the constitutive transcription of a gene as wasteful? These fulfil usually important roles... $\endgroup$ – Chris Jul 30 '16 at 16:37
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The phrase “background level” is generally used in an experimental context where you are measuring the effect on a variable of some agent or stimulus. You might expect that your control condition(s) — without the stimulus — would give you a measurement of zero, but often that is not the case, and one needs to subtract this control value — often referred to as ‘background’ — from the experimental value.

Such background values can be real — e.g. background radiation — or artefacts caused by fundamental limitations in the measuring system used. Generally the implication is that their origin is unknown or is not specific. That is why I think that the use of the term ‘background’ in the context of the lac operon is somewhat unfortunate. I would have used (and encourage you to use) the term ‘basal level’ instead*. As the text states, there is a low — but specific and necessary — expression of β-galactosidase in the non-induced situation, so that when lactose is added some of it can be converted to the inducer, allolactose, which will then result in the synthesis of the greater amount of β-galactosidase needed to deal with the rest of the lactose.

*Footnote

In fact I hate the term ‘level’ except for water in the bath tub, and I would have used ‘amount’ or ‘activity’, but that’s not the point at issue here.

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    $\begingroup$ Love the footnote of you answer... I also greatly dislike the word "level" because it is not very specific and can mean different things in different situations. Many individuals use the word "level" when they more accurately should be saying "concentration". $\endgroup$ – Vance L Albaugh Jul 30 '16 at 16:25
  • $\begingroup$ Concentration is a measure of the expression level. I, however, do concur that people use this word casually which would is likely to mislead others. "Background level" is not a suitable usage at all. $\endgroup$ – WYSIWYG Jul 31 '16 at 19:56

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