SPO13 is required for two
divisions in meiosis. In the absence of SPO13 , sporulating
diploids undergo a single equational MII-like division resulting in
dyads with diploid spores. When overexpressed in mitosis, SPO13
causes a CDC28 -dependent arrest at G2/M. Overexpression in
meiosis causes a transient arrest of MI and also restores two divisions
to cdc28-1 (which otherwise executes a single MI at a semi-permissive temperature). Together, these results suggest that
SPO13 promotes both MI and MII through interaction with the
CDC28 cell cycle machinery. Further analysis of SPO13
function has revealed that 1) the mitotic arrest and two-division
meiotic phenotypes are inseparable by mutation and are likely due to the
same activity. 2) Spo13p is a phosphoprotein localized to the nucleus in
mitotic cells. 3) Spo13p cannot accumulate in mitotic cdc28-1N
cells at 37 o or when CDC28 is inactivated in G1 with
alpha-factor. These observations demonstrate that accumulation of Spo13p
during overexpression in mitosis is CDC28 -dependent. They raise
the possibility that SPO13 both acts through, and is regulated
by, CDC28 activity in meiosis.
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