Next, we monitored the localisation of two proteins involved in transcriptional silencing, namely Sir3p and Rap1p, during chronological aging, because the redistribution of the Sir complex from the telomeres to the nucleolus has been previously observed during replicative aging (Sinclair et al. 1997).
The arrival of the sir complex in the longer-lived yeast strain repressed the genes from which the circles were generated and delayed aging.
In the absence of yKu, TPE is disrupted and less Sir3 and Sir4 are present in the subtelomeric regions [ 25- 27], indicating that yKu is required in order to facilitate either the recruitment of the Sir complex or the assembly of a silencing competent structure.
To further investigate the derepressor activity of Dot1 and to exclude possible telomere-looping effects [ 50], Dot1 was targeted to a mating-type locus internal to the chromosome, HMLα, in which the I-silencer element was inverted to allow spreading of the Sir complex to the neighboring URA3 reporter [ 51].
In addition, silencing also requires the catalytic activity of Sir2, a histone deacetylase acting on acetylated H4K16 (Imai et al. 2000; Landry et al. 2000), and the assembly of a repressive chromatin domain by interactions among multiple copies of the Sir protein complex throughout the domain (Rusche et al. 2002; Thurtle and Rine 2013).
But in the longer lived yeast, the sir complex left its usual position and migrated to the cell's nucleus, to a subcompartment called the nucleolus where the ribosomes, the cell's protein-making factories, are assembled.
Once recruited, Sir2 is thought to deacetylate the histones of adjacent nucleosomes allowing the Sir complex to spread outwards from the site of assembly [ 6, 7].
The average of Sir2, Sir3, and Sir4 was used for the effect of the Sir complex.
The presence of other histone modifications, such as histone variant H2A.Z, methylation of lysine residues 4 and 79 of histone H3 and the acetylation of lysine 16 in histone H4, may limit the spread of the Sir complex [ 8- 12].
The binding signals of the Sir complex and Set1 from ChIP-chip experiments [21] displayed the same patterns: intermediate binding affinity increased expression noise, whereas strong bindings were associated with low expression noise (Fig. 2F).
