Cell cycle programs of gene expression control morphogenetic protein localization

Genomic studies in yeast have revealed that one eighth of genes are cell cy cle regulated in their expression. Almost without exception, the significan ce of cell cycle periodic gene expression has not been tested. Given that m any such genes are critical to cellular morphogenesis, we wanted to examine the importance of periodic gene expression to this process. The expression profiles of two genes required for the axial pattern of cell division, BUD 3 and BUD10/AXL2/SRO4, are strongly cell cycle regulated. BUD3 is expressed close to the onset of mitosis. BUD10 is expressed in late G1. Through prom otor-swap experiments, the expression profile of each gene was altered and the consequences examined. We found that an S/G2 pulse of BUD3 expression c ontrols the timing of Bud3p localization, but that this timing is not criti cal to Bud3p function. In contrast, a G1 pulse of BUD10 expression plays a direct role in Bud10p localization and function. Bud10p, a membrane protein , relies on the polarized secretory machinery specific to G1 to be delivere d to its proper location. Such a secretion-based targeting mechanism for me mbrane proteins provides cells with flexibility in remodeling their archite cture or evolving new forms.