What are housekeeping and constitutive genes?

Question

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?

Answer 1

I think the best answer is both: they are continually transcribed at a rate that maintains the synthesized products at a (roughly) constant level. These terms are really just descriptive terms, there is no true biological dichotomy, it's just that some genes are expressed pretty much all the time and others are only produced when needed, for example genes for proteins needed at a particular stage of the cell cycle.

As @canadianer mentions in a comment, there are also other forms of post-transcriptional regulation, such that constitutive transcription doesn't necessarily mean constant production of protein at an equal rate.

Wikipedia has quite a list of different "housekeeping genes" and you will see the list is quite extensive. It certainly isn't true that all of these genes are expressed at exactly the same levels all the time, it is simply that their expression is consistent enough to be used as an approximate standard when comparing expression levels.

If I could produce a definition that reflects reality better than the ones you found, I would amend the Oxford definition to something like:

Constitutive Genes are genes that are expressed following interaction between a promoter and RNA polymerase at a level that doesn't seem to vary much across cells of the same type, doesn't require a very special transcription factor, and so far biologists haven't found an experimental condition that greatly impacts the expression level.

Answer 2

The questioner asks “What does ‘transcribed at a relatively constant level’ mean?” The answer is absolutely nothing!

A gene can be transcribed at a constant rate, or maintained at a constant level (in so far as ‘level’ — a word that should be avoided — is taken to mean ‘amount’). Perhaps it means ‘transcribed so as to maintain a relatively constant cellular concentration of the product’. It is unclear.

I’m afraid that it is a waste of time trying to find an authority for definitive meanings for terms that were coined in a period of incomplete scientific knowledge and have changed their meaning or importance as science has progressed and time has passed. Wikipedia is written by anyone who cares to contribute and its authority tends to vary with the amount of interest experts in the field have. For computing science you are on a winner, for molecular biology you may not be.

As far as the term housekeeping gene goes, a Google ngram shows that it appeared about 1980, at a time when little was known about the regulation of the expression of eukaryotic genes. The term expressed the simplistic idea of genes that were expressed continually (as in the examples quoted) as opposed to those induced by particular stimuli like sexual differentiation or perhaps change in diet. In fact little was known about inducable genes at the time, and the concept was very much based on an analogy with bacterial systems, which are different in many ways. It is still useful in a general sense, but no more.

PS

I've edited the Wikipedia entry. We'll see if anyone cares. (I can edit it, but anyone else can switch it back to the original. In that case it is best discussed.)