||It has been suggested that this article be merged into Sirtuin 1. (Discuss) Proposed since October 2012.|
Sir2 (whose homolog in mammals is known as SIRT1, SIR2L1 or Sir2α) was the first gene of the sirtuin genes to be found. It was found in budding yeast, and, since then, members of this highly conserved family have been found in nearly all organisms studied. Sirtuins are hypothesized to play a key role in an organism's response to stresses (such as heat or starvation) and to be responsible for the lifespan-extending effects of calorie restriction.
Nomenclature in various organisms
The three letter yeast gene symbol Sir stands for Silent Information Regulator while the number 2 is representative of the fact that it was the second SIR gene discovered and characterized. The term sirtuin is derived from Sir2 and stands for Silent Information Regulator Two (Sir2) protein.
The name Sir2 is used for the enzyme in the yeast Saccharomyces cerevisiae (where it was first discovered), in the fruit fly Drosophila melanogaster, while in the roundworm, Caenorhabditis elegans, Sir-2.1 is used to denote the gene product most similar to yeast Sir2 in structure and activity.
Method of action and observed effects
Sirtuins act primarily by removing acetyl groups from lysine residues within proteins in the presence of NAD+; thus, they are classified as "NAD+-dependent deacetylases" and have EC number 3.5.1. They add the acetyl group from the protein to the ADP-ribose component of NAD+ to form O-acetyl-ADP-ribose.
Sir2 is the only Class III histone deacetylase (HDAC) in budding yeast.' The HDAC activity of Sir2 results in tighter packaging of chromatin and a reduction in transcription at the targeted gene locus. The silencing activity of Sir2 is most prominent at telomeric sequences, the hidden MAT loci (HM loci), and the ribosomal DNA (rDNA) locus (RDN1) from which ribosomal RNA is transcribed.
Limited overexpression of the Sir2 gene results in a lifespan extension of about 30%, if the lifespan is measured as the number of cell divisions the mother cell can undergo before cell death. Concordantly, deletion of Sir2 results in a 50% reduction in lifespan. In particular, the silencing activity of Sir2, in complex with Sir3 and Sir4, at the HM loci prevents simultaneous expression of both mating factors which can cause sterility and shortened lifespan. Additionally, Sir2 activity at the rDNA locus is correlated with a decrease in the formation of rDNA circles. Chromatin silencing, as a result of Sir2 activity, reduces homologous recombination between rDNA repeats, which is the process leading to the formation of rDNA circles. As accumulation of these rDNA circles is the primary way in which yeast are believed to "age", then the action of Sir2 in preventing accumulation of these rDNA circles is a necessary factor in yeast longevity.
Starving of yeast cells leads to a similarly extended lifespan, and indeed starving increases the available amount of NAD+ and reduces nicotinamide, both of which have the potential to increase the activity of Sir2. Furthermore, removing the Sir2 gene eliminates the life-extending effect of caloric restriction. Experiments in the nematode Caenorhabditis elegans and in the fruit fly Drosophila melanogaster support these findings. As of 2006[update], experiments in mice are underway.
However, some other findings call the above interpretation into question. If one measures the lifespan of a yeast cell as the amount of time it can live in a non-dividing stage, then silencing the Sir2 gene actually increases lifespan  Furthermore, calorie restriction can substantially prolong reproductive lifespan in yeast even in the absence of Sir2.
In organisms more complicated than yeast, it appears that Sir2 acts by deacetylation of several other proteins besides histones.
Resveratrol is a substance that has been shown through experiment to have a number of life-extending and health benefits in various species; it also increases the activity of Sir2, which is the postulated reason for its beneficial effects. Resveratrol is produced by plants when they are stressed, and it is possible that plants use the substance to increase their own Sir2 activity in order to survive periods of stress. Although there is mounting evidence for this hypothesis, its validity is debated.
In mammals, SIRT1 (the mammalian homolog of Sir2) has been shown to deacetylate and thereby deactivate the p53 protein. SIRT1 also stimulates autophagy by preventing acetylation of proteins (via deacetylation), proteins required for autophagy as demonstrated in cultured cells and embryonic and neonatal tissues. This function provides a link between sirtuin expression and the cellular response to limited nutrients due to caloric restriction. Furthermore, SIRT1 was shown to de-acetylate and affect the activity of both members of the PGC1-alpha/ERR-alpha complex, which are essential metabolic regulatory transcription factors.
In the fruit fly Drosophilia melanogaster, the Sir2 gene does not seem to be essential; loss of a sirtuin gene has only very subtle effects. However, mice lacking the SIRT1 gene (the sir2 biological equivalent) were smaller than normal at birth, often died early or became sterile.
Seven sirtuins are known in mammals.
- SIRT1 (also known as Sir2α) is the mammal homolog of Sir2. Mice that overexpress SIRT1 show eight properties of calorie restriction, including low cholesterol, low blood glucose, and low insulin levels. They also show increased numbers of mitochondria in their neurons.
- SIRT2 is expressed mainly in the brain.
- SIRT3, SIRT4, and SIRT5 are active in mitochondria, the energy-producing organelles that are a part of every cell.
- SIRT6 is active in the nucleus of the cell.
- SIRT7 is active in the nucleolus, a compartment of the nucleus reserved for the assembly of ribosomes. Sirt7 has been shown to activate RNA polymerase I transcription.
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