Regulation of cyclin A mRNA in leech embryonic stem cells

Leech embryos undergo invariant patterns of cleavage to yield identifiable cells that have characteristic timings of cell division. To elucidate how these cell-specific differences in cell-cycle timing are regulated, we have isolated a leech cyclin A cDNA clone and determined the patterns of cyclin A mRNA localization in identified cells of Helobdella leech embryos. The intensity of cyclin A mRNA hybridization staining is invariant in most cells but fluctuates systematically in the embryonic stem cells. During the repeated cell divisions of the stem cells, the intensity of cyclin A mRNA staining is high during the early part and low during the later part of each cell cycle. The increased staining at the start of the cell cycle is not due to accumulation of newly synthesized cyclin A mRNA, since the pattern of staining is not affected by inhibition of transcription. Treatment with protease prior to hybridization equalizes the intensity of cyclin A mRNA staining throughout the cell cycle, presumably by the removal of blocking proteins. Thus, the levels of maternal cyclin A mRNA remain constant during cell cycles of the embryonic stem cells, but accessibility of cyclin A mRNA to the hybridization probe fluctuates during each cell cycle. We suggest that the events underlying the fluctuating accessibility of cyclin A mRNA to the hybridization probe may be involved in regulating the translational availability of cyclin A mRNA throughout the cell cycle or protecting nascent cyclin A protein from degradation during the destruction period.