A form of position dependent gene repression
in yeast, known as silencing, requires the function of the silent
information regulators, Sir2p, Sir3p, and Sir4p. Position dependent
repression is associated with the cryptic mating-type cassettes at
HML and HMR , as well as telomeres and rDNA.
Immunolocalization studies have shown Sir3p and Sir4p to reside
predominantly at telomeres near the periphery of the nucleus. We have
found that exposure of yeast cells to the DNA alkylating agent methyl
methane sulfonate (MMS), results in a loss of telomeric silencing. Using
indirect immunofluorescence we found that Sir3p redistributes in
response to MMS. Sir3p requires entry into the S phase of the cell cycle
for redistribution and undergoes a time dependent change in its staining
pattern. Early in S phase Sir3p delocalizes from telomeres. Over time
Sir3p reorganizes into a brightly staining nuclear spot and then into
multiple (typically 10-50) spots throughout the nucleus that are not
reminiscent of telomeric localization. The redistribution is not a
consequence of cell cycle progression per se as cells
synchronized but not treated with MMS do not show delocalization at any
time during S phase. Continuation through S phase is not required for
the intitial delocalization because cells arrested with the S phase
inhibitor hydroxyurea still exhibit Sir3p delocalization near the G1/S
phase transition. We postulate a role for Sir proteins in the repair of
damaged DNA.
Return to YGM 1998 Abstract Index