Mismatch repair proteins collaborate with methyltransferases in the repair of O(6)-methylguanine.

DNA repair is essential for combatting the adverse effects of damage to the genome. One example of base damage is O(6)-methylguanine (O(6)mG), which stably pairs with thymine during replication and thereby creates a promutagenic O(6)mG:T mismatch. This mismatch has also been linked with cellular toxicity. Therefore, in the absence of repair, O(6)mG:T mismatches can lead to cell death or result in G:C-->A:T transition mutations upon the next round of replication. Cysteine thiolate residues on the Ada and Ogt methyltransferase (MTase) proteins directly reverse the O(6)mG base damage to yield guanine. When a cytosine is opposite the lesion, MTase repair restores a normal G:C pairing. However, if replication past the lesion has produced an O(6)mG:T mismatch, MTase conversion to a G:T mispair must still undergo correction to avoid mutation. Two mismatch repair pathways in E. coli that convert G:T mispairs to native G:C pairings are methyl-directed mismatch repair (MMR) and very short patch repair (VSPR). This work examined the possible roles that proteins in these pathways play in coordination with the canonical MTase repair of O(6)mG:T mismatches. The possibility of this repair network was analyzed by probing the efficiency of MTase repair of a single O(6)mG residue in cells deficient in individual mismatch repair proteins (Dam, MutH, MutS, MutL, or Vsr). We found that MTase repair in cells deficient in Dam or MutH showed wild-type levels of MTase repair. In contrast, cells lacking any of the VSPR proteins MutS, MutL, or Vsr showed a decrease in repair of O(6)mG by the Ada and Ogt MTases. Evidence is presented that the VSPR pathway positively influences MTase repair of O(6)mG:T mismatches, and assists the efficiency of restoring these mismatches to native G:C base pairs.

Architectural dynamics of the meiotic spindle revealed by single-fluorophore imaging.

Bipolarity of the meiotic spindle, required for proper chromosome segregation, is maintained throughout cell division despite rapid microtubule turnover. How this is achieved has remained mysterious, as determining the organization of individual spindle microtubules has been difficult. Here, we develop single-fluorophore speckle imaging to examine microtubule organization in the vertebrate meiotic spindle. We find that the mean length of microtubules is approximately 40% of spindle length. Long and short filaments distribute randomly throughout the spindle and those in close proximity can move in the same direction with highly heterogeneous velocities. The ratio between microtubule and spindle lengths remains unchanged as spindles elongate upon dynein-dynactin inhibition. However, maintaining this ratio depends on proper kinesin-5 function. Our data suggest that force transmission within the spindle must be understood in terms of the crosslinking dynamics of a tiled array of individual filaments, most of which do not span the distance from the pole to the metaphase plate.