AbstractSIRT7 is involved inrDNA transcription was made more clear when its localization was detected innucleolus.
Nucleolus is formed by looping of five different chromosomes (13,14, 15, 21 and 22)on which rDNA genes are present and are transcribed by RNAPOLI.RNA POLI and SIRT7 are located together in active genes of rDNA.tRNA and5sRNA are transcribed by RNA POLIII involving TFIIIC2 and interaction studiessuggested that SIRT7 is also an interacting partner of all these components. However, SIRT7 has anindirect effect on production of protein as its overexpression had no effect onit.
Hence, all these studiesgive a good idea that SIRT7 has a prominentregulating effect on the functions of both RNA POLI and RNA POLIII. Here wepresent a detailed review report of SIRT7 involvement in rDNA transcription andprotein synthesis and its role in cellular stress. IntroductionEpigenetics participates many of the biologicalprocess regulation and hence it is now greatly studied. Since it is inheritableand can be moved backward it is easily accepted by the biological system.
Epigenetics constitutes post translational modification as the main machineryfor regulating expression of gene among which DNA methylation, remodeling ofchromatin and histone acetylations anddeacetylations are few mechanism, histones are the exclusive target of thesephenomenon for both inhibiting and activating gene in particular to carry out abiological process. Histone deacetylases (HDAC’s) are the enzyme which carry outdeacetylations of acetylated histones by removing acetyl groups from an ?-N-acetyl lysine amino acid on a histone making it more compact and unavailable forany molecular process. Sub-families of HDAC are class1-class4 and here we willconcentrate on a class3 HDAC called sirtuins7. Unlike all other HDAC’s sirtuinshas a NAD+ binding catalytic domain andwidely distributed in nucleus (SIRT1,2,6,7), cytoplasm (SIRT1,2), mitochondria(SIRT3,4,5).Amongst all the sirtuins sirt1 and sirt7 is involved in variousmetabolic and cellular functions like energy metabolism, DNA damage, regulatesfat oxidation in liver, ageing in lower vertebrates, in higher vertebratesinspite of directly involved in ageing regulation it is regarded as a metabolicfactor. Its localization in nucleolus indicates its involvement in ribosomebiogenesis or protein synthesis and this has been proved by various researchersbecause ribosomes are found in close proximity with the nucleolar region. Sirt7 is located onchromosome 17 in subtelomeric region containing 10 exons and an ORF of 1203bpencoding a protein length of 400 amino acids. It has a molecular weight of 44KDand its catalytic domain is present between 90 and 331 amino acid residues.
Innormal conditions it is highly expressed in metabolically active tissues likeCD-33 positive myeloid bone marrow precursor cells, liver and almost alltissues except in tissues which don’t divide like muscle, brain and ovary. Likeall other sirtuins Sirt7 has a weak deacetylase activity, some of the knownsubstrates are p53, h3k18ac, NPM1, PAF53,GABPbeta1. table1. ShowingInteracting partners and substrates of SIRT7 Our focus here isPAF53 substrate the deacetylase activity of SIRT7 is also involved in rDNAtranscription. RNA polymerase1 is responsible for rDNA transcription which isrequired for synthesis of ribosome.
Metabolically active cells are in need ofcontinuous ribosome synthesis and function for which Sirt7 deacetylates PAF53(at position k373) a subunit of RNA polymerase1 this activity of SIRT7 isrequired for RNA pol1 binding to rDNA promoter for transcription initiation andelongation2. SIRT7 and rDNA transcription Ribosomal RNA issynthesized in nucleolus which contains rDNA genes in clusters of fivedifferent chromosomes3.RNAse treatment caused release of SIRT7from nucleolus to nucleoplasm suggested that RNA is required for SIRT7retention in nucleolus and thereby carrying out rDNA transcription byassociating with RNA POLI.
indicating association of SIRT7 with RNA plymerase1during transcription in nucleolus. Co-immunoprecipitation assays withantibodies against RNA polymerase1 but without control antibodiesco-precipitated endogenous SIRT712.Likewise,when RNA Polymerase1 co-precipitated with alpha FLAG(M2) antibody against FLAGtagged SIRT7 clearly indicated that SIRT7 interacts with RNA polymerase1machinery to carry out transcription12.Knockdown of SIRT7 caused significant decrease in rRNA synthesis followed bylesser association of RNA polymerase1 to the rDNA promoter and overexpressioncaused increase in RNA ploymerase1 mediated transcription4. This clearly indicates functional role of SIRT7 in rDNA transcriptionbut clear functional characterization of SIRT7 is not specified.
As we know histonedeacetylase do not have direct DNA binding domain requiring additional proteinwith a DNA binding domain. Hence, reports say that UBF which binds to UCE of promoter and recruits other factors needed forforming complex of pre-initiation like SL1, SL1 consists of TATA bindingprotein(TBP) and TAF’s was confirmed when sertinol inhibitor repressed thetranscription process5.It is reported that SIRT7 is foundassociated with UBF in most cell cycle stages(G1, G1/S, Sphase) and even duringmitosis SIRT7 and UBF are complexed and remain bound to chromatin instead ofresiding in NOR’s.SIRT7 deacetylates PAF53a subunit of RNA polymerase1 complex this association of SIRT7 and PAF53 isessential for recruitment of RNA polymerase1 on rDNA promoter by interactingwith other subunits and UBF6. Co-immunoprecipitation experimentsconfirmed the association of SIRT7 and PAF53.The association of PAF53 with RNAPOLI was not affected by both wild type and mutated PAF53(K373) suggesting thatacetylation does not affect interaction of PAF53 with POLI.MutatedPAF53(arginine in place of lysine) revealed high pre-rRNA synthesis indicatinghypoacetylation is important for rRNA synthesis6.Duringstress, when SIRT7 is released from nucleoli rDNA transcription is stopped as aresult of lost interaction between SIRT7 and PAF53 and thereby hyperacetylationof PAF53.6.Pre-rRNA synthesis was reduced in bothcases when CBP and PAF53 was knocked down separately suggesting that bothacetylation and deacetylation by SIRT7 is necessary for rRNA synthesis.