What are housekeeping and constitutive genes?

[Tyto alba]

I have come across two set of definitions which are not contradictory but different.

From wikipedia:

A constitutive gene is a gene that is transcribed continually as opposed to a facultative gene, which is only transcribed when needed.


A housekeeping gene is typically a constitutive gene that is transcribed at a relatively constant level. The housekeeping gene's products are typically needed for maintenance of the cell. It is generally assumed that their expression is unaffected by experimental conditions. Examples include actin, GAPDH and ubiquitin.

My question from these definitions: What does 'transcribed at a relatively constant level' mean? Does it mean that a constant level of gene product is maintained in a cell? I can't understand there meanings atal.

Again,according to Oxford dictionaries

Constitutive Genes are genes that are expressed following interaction between a promoter and RNA polymerase without additional regulation. (Oxford Dictionary of Biochemistry and Molecular Biology)


Housekeeping genes - Constitutive loci that are theoretically expressed in all cells in order to provide the maintenance activities required by all cells: e.g., genes coding for enzymes of glycolysis and the and the citric acid cycle.(Dictionary of Genetics)

These definitions do not tell whether they are continually transcribed or the synthesised products are present in constant amount.

Which one best define them?

 

[Ygggdrasil]

Because proteins are degraded at some rate and diluted through cell division, if you transcribe and translate a gene at a constant rate, you will achieve some relatively constant steady state concentration of the protein inside of the cells (at least assuming the degradation rate is not variable over time).

In practice, researchers use housekeeping genes as an internal standard in many experiments. If we want to see whether a gene changes in expression in a particular experiment, we usually measure the concentration of our target protein relative to the concentration of the housekeeping gene. This helps to normalize differences in the total amount of cells analyzed in many experiments. So, we're more concerned that the concentration of the housekeeping gene stays relatively constant inside the cell across a number of conditions rather than the rate of transcription remains constant.

 

[Tyto alba]

If we want to see whether a gene changes in expression in a particular experiment, we usually measure the concentration of our target protein relative to the concentration of the housekeeping gene. This helps to normalize differences in the total amount of cells analyzed in many experiments. So, we're more concerned that the concentration of the housekeeping gene stays relatively constant inside the cell across a number of conditions rather than the rate of transcription remains constant.

I guess you meant housekeeping gene products(proteins)?

 

[Ygggdrasil]

Typically, we use housekeeping genes to normalize expression when looking at mRNA levels (e.g. in RT-qPCR experiments) or protein levels (e.g. in immunoblotting experiments). So, they should have relatively constant concentration of mRNA/cell and protein/cell.

 

[epenguin]

Don't get stuck or hung up on definitions, just read on and see what they talk about when they use the words. Not like formally constraining logical or mathematical definitions.

'Constitutive' is more phenomenological - describing observation, 'housekeeping' is more explicative in terms of biological function. The terms, especially the second, are a fuzzy-edged.

'Constitutive'/ 'induced' are maybe more precise. The terms were coined by the distinguished French scientist, Monod. He was distinguished for coining terms. Before he did that 'inducible' enzymes were referred to as 'adaptive' enzymes. An example was beta-galactosidase, the enzyme that hydrolyses lactose allowing E coli cells to metabolise and grow on it. In the absence of lactose they did not synthesise this enzyme, in fact lactose 'repressed' beta-galactosidase formation. This was clearly adaptive, the time being only expressed when there was something useful for it to do. In contrast to other enzymes that were useful all the time, and synthesised all the time, i.e. 'constitutive'. However as the system was not always 'adaptive' in that it could be fooled by a number of non-metabolisable sugars or analogs that interacted with the repressor protein like like lactose did and induced the enzyme (caused the cell to synthesise it), Monod changed the terminology.

In higher organisms the proteins needed to keep all cells going, like the glycolytic enzymes, will be 'housekeeping'; those made only in specialised cells not. E.g. genes coding for rhodopsin or haemoglobins would not be called housekeeping genes.

There is no implication of very detailed prescriptions, e.g. a housekeeping gene will not necessarily be synthesised at a constant rate throughout the cell cycle.