The HMG and A24 proteins are major structural chromosomal proteins whose functions are as yet unknown, although there is evidence suggesting that the HMG proteins may be associated with structurally active regions of chromatin and that the amount of A24 drops as cells become more active. We have recently found that yeast contains HMG; indirect evidence suggests that A24 is present as well. We propose to study HMG during the cell cycle of the fission yeast S. pombe. It has been reported that, during mitosis, the chromatin of S. pombe condenses, in contrast to what occurs in the budding yeast, S. cerevisiae. There are at present 57 known temperature sensitive mutants of S. pombe. At 35 degrees C the cells are blocked at one of a number of different places in the cell cycle, the place depending on the mutant grown. This makes it possible to grow large batch cultures of these mutants and study HMG at defined cell cycle positions. We will also try to get definitive evidence for the presence of A24. If it is verified to be present, similar studies of A24 will be made.