M phase-specific activation of the Nicotiana sylvestris Cyclin B1 promoterinvolves multiple regulatory elements

B-type cyclins are cell cycle regulatory proteins specifically expressed in G2-M phases. To understand the mechanisms that regulate their expression, it is important to identify the required promoter element(s). By using sync hronized BY-2 cell cultures stably transformed with chimeric genes composed of sequential Nicsy; CycB1; 1 promoter deletions fused to the beta-glucuro nidase reporter gene (gus), we show that at least five distinct promoter re gions are required for maximal M-phase expression. Furthermore, two distinc t promoter regions contain sufficient element(s) to induce M-phase-specific expression. In one of these regions, a 23 bp promoter element is able to a ctivate reporter gene expression in cells induced to divide in an orientati on-independent manner but without an M-phase specific expression. Therefore , this 23 bp element, which contains a 5 bp element identical to the MYB bi nding core, is a good upstream activating sequence (UAS) candidate. Moreove r, electrophoretic mobility shift assays show that this putative UAS specif ically binds protein complexes that appear to differ whether cells are cycl ing or not. The constitutive transcriptional activation mediated by this UA S would suggest that the Nicsy; CycB1; 1 gene promoter is regulated through an activation/repression mechanism.