Telomere end processing in S. cerevisiae

Click a step to render it. One fork passage makes two daughter ends, so the top of the chart is a split rather than a decision and both tracks are live; they converge at step 4. One decision point below that, where the long-telomere branch skips elongation and rejoins at fill-in.

LEADING-STRAND END LAGGING-STRAND END END PROCESSING G-STRAND ELONGATION C-STRAND FILL-IN CAPPED LATE S LATE S → G2 G2/M → G1 replication fork passes: two daughter ends no incision step: 5-prime end already recessed both ends converge here D1Rif2 load low? yes (short) no (long) rejoins fill-in no telomerase lose one cap element added repeats restore the Rif2 load next S phase 1 Leading synthesis: blunt terminus 2 Sae2-P activates Mre11 endonuclease 3 Bidirectional resection 4 Cdc13 alone loads on the overhang 5 MRX retained; Tel1 phosphorylates Cdc13 6 Est1 bridges TLC1; Est2 copies template 7 Pif1 and Ipl1 remove telomerase 8 Stn1-Ten1 join; Pol alpha-primase fills in 9 Capped: Rap1, CST, Ku all bound 1* Lagging synthesis: 3-prime overhang 3* Unidirectional resection A No telomerase: repeats erode, ends fuse B Long telomere: Rif2 takes the socket C CST lost: resection, checkpoint, arrest D Rap1 depleted: fusion at full length E Ku lost: long tails, short telomeres

unique sequence U U U U U U U U U U U U U U U U U U U U U U U U U U U U G C G C G C G C G C G C G C G C G C G C G C G C G C G C 3′ 5′ Rap1 Rif1 Rif2 Sir2-3-4 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Ku Mre11 Rad50 Xrs2 Xrs2 C-term replisome, Pol ε cap Rap1 no duplex CST no overhang Ku bound blunt terminus Rif2 load maximal → MRX residence brief the fork arrives from a subtelomeric origin: no primer is laid at the telomere on this strand new G-strand runs to the terminus, so the daughter end is blunt and carries no overhang duplex TG1-3 array, Rap1 ~1 per 18 bp
unique sequence U U U U U U U U U U U U U U U U U U U U U U U U U U U U G C G C G C G C G C G C G C G C G C G C G C G C G C G C 3′ 5′ Rap1 Rif1 Rif2 Sir2-3-4 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Ku RNA RNA RNA RNA RNA Pol α-primase Pol δ cap Rap1 no duplex CST none established Ku bound Rad27 Primers removed Cdc9 nicks sealed terminal primer removed, no 3′ end to fill behind it 3′ overhang, unfilled each fragment is primed distally and extended leftward until it meets the one before it no incision and no MRX arm needed here; this end already has a recessed 5′ terminus
unique sequence U U U U U U U U U U U U U U U U U U U U U U U U U U U U G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C 3′ 5′ Rap1 Rif1 Rif2 Sir2-3-4 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Ku Mre11 Rad50 Xrs2 Xrs2 C-term cap Rap1 full array CST no overhang Ku bound Cdk1 Sae2 P S267 phosphorylates activates Sae2-P is a cofactor here, not the nuclease incision, internal to the end
unique sequence U U U U U U U U U U U U U U U U U U U U U U U U U U U U G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C 3′ 5′ Rap1 Rif1 Rif2 Sir2-3-4 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Ku Mre11 Rad50 Xrs2 Xrs2 C-term cap Rap1 full array CST no overhang Ku bound RPA RPA RPA Exo1 Sgs1-Dna2 Mre11: 3′ → 5′, toward the end Exo1 and Sgs1-Dna2: 5′ → 3′ on the C-strand, travelling inward RPA holds the strand until Cdc13 takes it in panel 4: no CST here, and none needed yet
unique sequence U U U U U U U U U U U U U U U U U U U U U U U U U U U U G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C 3′ 5′ Rap1 Rif1 Rif2 Sir2-3-4 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Ku Mre11 Rad50 Xrs2 Xrs2 C-term cap Rap1 full array CST none established Ku bound RPA RPA RPA Exo1 Sgs1-Dna2 Exo1 and Sgs1-Dna2: 5′ → 3′ on the C-strand, travelling inward no incision here: the 5′ end the nucleases load on was left by primer removal MRX loads Exo1 and Sgs1-Dna2 without cutting, then leaves
unique sequence U U U U U U U U U U U U U U U U U U U U U U U U U U U U G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C 3′ 5′ Rap1 Rif1 Rif2 Sir2-3-4 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Ku Mre11 Rad50 Xrs2 Xrs2 C-term Cdc13 cap Rap1 short array CST Cdc13 only Ku bound RPA RPA RPA handed off 3′ G-overhang occupancy is the capping event; Stn1-Ten1 are drawn only at fill-in
unique sequence U U U U U U U U U U U U U U U U U U U U U U U U U U U U G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C 3′ 5′ Rap1 Rif1 Rif2 Sir2-3-4 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Ku Mre11 Rad50 Xrs2 Xrs2 C-term Rif2 Tel1 Cdc13PT308 RPA RPA cap Rap1 short array CST Cdc13 only Ku bound promotes elongation Cdk1 phosphorylates T308 short telomere: fewer Rap1, less Rif2, so the Xrs2 C-terminus wins the socket and Rif2 is kept out T308 is a Cdk1 site and is drawn; S249 and S255 are the contested Tel1 sites and are not frames 1 and 2 are drawn in sequence; no evidence establishes that order
unique sequence U U U U U U U U U U U U U U U U U U U U U U U U U U U U G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C 3′ 5′ Rap1 Rif1 Rif2 Sir2-3-4 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Ku Mre11 Rad50 Xrs2 Xrs2 C-term Tel1 Cdc13PT308 RPA RPA RPA RPA RPA cap Rap1 short array CST Cdc13 only Ku bound TLC1 Est2 Est3 Est1 non-processive: a minority of telomeres is extended per cycle, and the shortest preferentially Cdc13-T308 recruits and stays; telomerase leaves it behind as it extends repeats added
unique sequence U U U U U U U U U U U U U U U U U U U U U U U U U U U U G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C 3′ 5′ Rap1 Rif1 Rif2 Sir2-3-4 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Ku Cdc13PT308 RPA RPA RPA RPA RPA cap Rap1 short array CST Cdc13 only Ku bound TLC1 Est2 Est3 Est1 Pif1 Ipl1 remove Pif1 and Ipl1 take different contacts; removing the enzyme frees the terminal repeat
unique sequence U U U U U U U U U U U U U U U U U U U U U U U U U U U U G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C 3′ 5′ Rap1 Rif1 Rif2 Sir2-3-4 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Ku Cdc13SUK909Stn1Ten1PT223/S250Est1 Cdk1 phosphorylates Stn1 Cdc13PT308 RPA RPA RPA RPA RPA cap Rap1 short array CST assembled Ku bound Pol α-primase Stn1-Ten1 block the Cdc13-Est1 contact primer internal to the footprint new C-strand restores Rap1 sites, and with them the Rif1 / Rif2 load CST holds the terminal footprint; the polymerase runs inward and Ku follows the new junction
unique sequence U U U U U U U U U U U U U U U U U U U U U U U U U U U U G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C 3′ 5′ Rap1 Rif1 Rif2 Sir2-3-4 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Cdc13SUK909Stn1Ten1PT223/S250 Ku duplex TG1-3 array residual 3′ tail cap Rap1 full array CST assembled Ku bound the cap is these three bound at once, on the two substrates and the junction between them click any glowing element to see what its loss does
unique sequence U U U U U U U U U U U U U U U U U U U U U U U U U U U U G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C 3′ 5′ Rap1 Rif1 Rif2 Sir2-3-4 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Cdc13SUK909Stn1Ten1PT223/S250 Ku Mre11 Rad50 Xrs2 Xrs2 C-term Mre11 Rad50 Xrs2 Xrs2 C-term Mre11 Rad50 Xrs2 Xrs2 C-term U U U U U U U U U U U U U U U U U U second chromosome end, repeats already eroded Dnl4-Lif1 Nej1 ligates cap Rap1 full array CST assembled Ku bound tlc1Δ (or est2Δ) no telomerase, so every division takes repeats off and nothing puts them back the cap has lost no element here; it has run out of the sequence all three bind
unique sequence U U U U U U U U U U U U U U U U U U U U U U U U U U U U G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C 3′ 5′ Rap1 Rif1 Rif2 Sir2-3-4 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Ku Mre11 Rad50 Xrs2 C-term displaced Rif2 Cdc13 RPA RPA cap Rap1 full array CST Cdc13 only Ku bound 3′ G-overhang long telomere: five Rap1 boxes and five Rif2 against three in panel 5, so Rif2 wins the socket the Xrs2 C-terminus is held out, so no pole, no Tel1 flag, and no T308 on Cdc13
unique sequence U U U U U U U U U U U U U U U U U U U U U U U U U U U U G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C 3′ 5′ Rap1 Rif1 Rif2 Sir2-3-4 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Cdc13SUK909Stn1Ten1PT223/S250 Ku RPA RPA RPA RPA RPA RPA RPA RPA RPA RPA RPA RPA RPA RPA RPA RPA RPA RPA RPA RPA RPA Exo1 5′ strand degraded past the repeats cap Rap1 full array CST assembled CST CST lost Ku bound cdc13-1 at 37 °C (or stn1-13, ten1-31) Mec1 Ddc2 Rad9 activates Rad53 P phosphorylatesmetaphase arrest no Cdc13 on the overhang, so nothing stops resection at the array the substrate that marks the lost cap is the substrate that starts the checkpoint
unique sequence U U U U U U U U U U U U U U U U U U U U U U U U U U U U G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C 3′ 5′ Rap1 Rif1 Rif2 Sir2-3-4 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Cdc13SUK909Stn1Ten1PT223/S250 Ku RPA RPA RPA Mre11 Rad50 Xrs2 Xrs2 C-term 3′ overhang, C strand receded G C G C G C G C G C G C G C second telomere, full length, Rap1 also depleted Dnl4-Lif1 Nej1 ligates cap Rap1 full array Rap1 array lost CST assembled Ku bound NHEJ NHEJ degron induced rap1 degron (or rap1-td) the array keeps every repeat; the protein goes and the C strand gives ground behind a fixed 3′ end fusion here needs Ku, which is the element branch E removes
unique sequence U U U U U U U U U U U U U U U U U U U U U U U U U U U U G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C G C 3′ 5′ Rap1 Rif1 Rif2 Sir2-3-4 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Rap1 Rif1 Rif2 Cdc13SUK909Stn1Ten1PT223/S250 Ku RPA RPA RPA RPA RPA RPA RPA RPA RPA Exo1 Sgs1-Dna2 cap Rap1 full array CST assembled Ku bound Ku Ku lost yku70Δ (or yku80Δ) shifted to 37 °C Mec1 Ddc2 Rad9 activates Rad53 P phosphorylatesarrest no Ku at the junction, so resection is not restrained and the tail is long outside late S as well telomeres are also shorter, and TLC1 leaves the nucleus CST still holds the overhang at either temperature; what changes is whether the tail crosses the threshold

    Arrow keys move through the leading-strand route and the shared tail below the merge. The lagging-strand track, 1* and 3*, is reached by clicking. Space plays or pauses an animation. D1 is a decision point, not a panel, and the bar at the top is a split rather than a decision: both arms always happen.