Ribonukleotid-reduktaza (RNR) je tetramerni proteinski kompleks koji katalizira konverziju nukleotida u dezoksinukleotide, korak koji ograničava brzinu u biosintezide novo dezoksiribonukleotida i ima bitnu ulogu u replikaciji i popravku DNK . Uravnotežena opskrba dezoksiribonukleozid trifosfatima (dNTP) potrebna je za precizno dupliranje genoma. I ukupna koncentracija i ravnoteža između pojedinih dNTP (dATP, dTTP, dGTP i dCTP) strogo su regulirani ribonukleotid-reduktazom.[4][5] Aktivnost ribonukleotid-reduktaze je periodična, tokom ćelijskog ciklusa, raste od početnog niskog nivoa do maksimuma u ranoj S-fazi, a zatim opada na njenom kraju.[6][7]
Ribonukleotid-reduktaza sastoji se od dvije velike i dvije male podjedinice. U Saccharomyces cerevisiae, glavna izoforma velike podjedinice kodirana je RNR1, a druga izoforma RNR3; dvije male podjedinice kodirane su RNR2 i RNR4.[8] Homodimer Rnr1p: Rnr1p sadrži regulatorna i katalitska mjesta, a u heterodimeru Rnr2p: Rnr4p smješten je esencijalni kofaktor diferno-tirozil radikala . Ključna uloga Rnr4p je pravilno savijanje i stabilizirati Rnr2p koji pohranjuje radikale, formirajući stabilan kompleks Rnr2p/Rnr4p u omjeru 1: 1. Doprinos RNR3 redukciji ribonukleotida nije jasan. RNR3 se ne eksprimira tokom normalnog rasta, ali kao i ostale tri podjedinice snažno je induciran oštećenjem DNA, iako nikada ne doseže više od jedne desetine nivoa Rnr1p. Tokom većeg dijela ćelijskog ciklusa, Rnr1p i Rnr3p su lokalizirani u citoplazmi, dok su Rnr2p i Rnr4p prisutni u dru. Kao odgovor na S fazu ili oštećenje DNK, potkompleks Rnr2p: Rnr4p prolazi kroz redistribuciju jedra do citoplazme ovisne o kontrolnoj tački i veže homodimer Rnr1p, formirajući aktivni kompleks RNR. Dif1p kontrolira subćelijsku lokalizaciju potkompleksa Rnr2p: Rnr4p vezujući se izravno za njega i posredujući u njegovom jedarnom unosu. Wtm1p djeluje kao jedarno sidro za održavanje jedarne lokalizacije Rnr2p: Rnr4p izvan S-faze ili u odsustvu oštećenja DNK.[9][10][11]
Inhibicija aktivnosti ribonukleotid-reduktaze tretmanom hidroksiureje rezultira zaustavljanjem ćelijskog ciklusa S-faze i velikim pupoljcima, jednostrukim ćelijama. I RNR1 i RNR2 su bitni za održivost, dok RNR3 nije.[12][13][14][15]
Aleli RNR1 i RNR2 osjetljivi na temperaturu zaustavljaju se s pupoljkom, cdc terminalnim fenotipom na temperaturi koja nije permisivna. Prekomjerna ekspresija RNR3 potiskuje smrtonosnost nultih mutacija rnr1. Deletirane ćelije za RNR3 preosjetljive su na rapamicin plus MMS. Delecija RNR4 je u nekim sojevima smrtonosna, ali u drugima nije, a ta se smrtnost može suzbiti prekomjernom ekspresijom RNR1 i RNR3 ili RNR2. Neki nulti mutanti rnr4 pokazuju spor rast i osjetljivost na mutagene, uključujući UV svjetlost i psoralene, kao i povećanu osjetljivost na oksidativni stres.[16][17][18]
Nulte mutirane ćelije rnr4 povećane su i također pokazuju veću učestalost pupanja, što ukazuje na kašnjenje mitoze/citokineze.
RNR je identificiran kod E. coli, biljaka i sisara. Budući da je aktivnost RNR presudna za brzo dijeljenje ćelija, njena prekomjerna ekspresija može dovesti do neoplazijske transformacije, što RNR čini metom za terapiju karcinoma. U ćelijama sisara, mala podjedinica RNR mjesto je djelovanja nekoliko antitumorskih sredstava, uključujući hidroksiureu i 4-metil-5-amino-1-formilizohinolin tiosemikarbazon (MAIQ).[19][20][21][22][23][24][25][26][27][28][29][30][31][32]
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^Zhang Z, et al. (2006) Nuclear localization of the Saccharomyces cerevisiae ribonucleotide reductase small subunit requires a karyopherin and a WD40 repeat protein. Proc Natl Acad Sci U S A 103(5):1422-7 PMID 16432237
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^Rittberg DA and Wright JA (1989) Relationships between sensitivity to hydroxyurea and 4-methyl-5-amino-1-formylisoquinoline thiosemicarbazone (MAIO) and ribonucleotide reductase RNR2 mRNA levels in strains of Saccharomyces cerevisiae. Biochem Cell Biol 67(7):352-7 PMID 2675933
^Chabes A, et al. (2000) Yeast ribonucleotide reductase has a heterodimeric iron-radical-containing subunit. Proc Natl Acad Sci U S A 97(6):2474-9 PMID 10716984
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^Sommerhalter M, et al. (2004) Structures of the yeast ribonucleotide reductase Rnr2 and Rnr4 homodimers. Biochemistry 43(24):7736-42 PMID 15196016
^Basso TS, et al. (2008) Low productivity of ribonucleotide reductase in Saccharomyces cerevisiae increases sensitivity to stannous chloride. Genet Mol Res 7(1):1-6 PMID 18273813
^Xu H, et al. (2006) Structures of eukaryotic ribonucleotide reductase I define gemcitabine diphosphate binding and subunit assembly. Proc Natl Acad Sci U S A 103(11):4028-33 PMID 16537480
^Wang PJ, et al. (1997) Rnr4p, a novel ribonucleotide reductase small-subunit protein. Mol Cell Biol 17(10):6114-21 PMID 9315671
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^Li B and Reese JC (2001) Ssn6-Tup1 regulates RNR3 by positioning nucleosomes and affecting the chromatin structure at the upstream repression sequence. J Biol Chem 276(36):33788-97 PMID 11448965
^Huang M and Elledge SJ (1997) Identification of RNR4, encoding a second essential small subunit of ribonucleotide reductase in Saccharomyces cerevisiae. Mol Cell Biol 17(10):6105-13 PMID 9315670
^Strauss M, et al. (2007) RNR4 mutant alleles pso3-1 and rnr4Delta block induced mutation in Saccharomyces cerevisiae. Curr Genet 51(4):221-31 PMID 17287963
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Nucleolus organizer regions are chromosomal regions crucial for the formation of the nucleolus, located on the short arms of the acrocentric chromosomes 13, 14, 15, 21 and 22
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