Expression patterns of a cyclin gene and cell cycle control in Nicotiana tabacum

Cyclins play a critical role in cell cycle progression. The cyclin genes are transcriptionally regulated and their protein products accumulate in specific phases of the cell cycle to activate cyclin-dependent kinases (CDK) by forming cyclin/CDK complexes, which serve to regulate entry into and exit from the cell cycle. In this study, we intended to elucidate the molecular mechanisms of cell cycle regulation underlying plant development by analyzing expression patterns of a tobacco mitotic cyclin gene, CycB1, during growth and upon hormone treatments. In addition, the transcriptional regulation of CycB1 was studied using transgenic plants.; We have isolated and sequenced a cyclin genomic DNA clone, CycB1, from Nicotiana tabacum cv. Xanthi. To investigate the temporal and spatial expression patterns of CycB1, the CycB1 promoter was fused to the GUS reporter gene, and GUS activity was examined in stably transformed tobacco plants. Strong GUS expression was observed in apical meristems, axillary meristems, root tips, lateral root initiation zones and primordia, cambium tissues within stems, young leaves, young flowers, and immature seeds. The GUS activities were consistent with the expression profile of the endogenous CycB1 mRNA. Furthermore, during seed germination, the GUS activity was correlated with rapid cell divisions in the cotyledons. Also, the growth activities of axillary buds were linked to the expression level of CycB1. In response to exogenous auxin treatment, GUS activity driven by the CycB1 promoter was strongly induced in lateral root primordial of seedlings.; To further determine the quantitative and qualitative control of CycB1, transcription, we analyzed GUS activities in transgenic lines with the GUS reporter gene driven by various fragment lengths of the 5′ CycB1 promoter region. Our data showed that multiple regulatory sequences between +26bp and –413bp upstream of CycB1, transcription initiation site were necessary for proper gene expression. In addition, the sequence between –770bp to –413bp might control the quantitative transcription of CycB1 in meristem tissues. Altogether, theses studies demonstrate that the transcriptional activation of CycB1 is correlated with cell proliferation, and that CycB1 is expressed in a wide range of cell types at early plant developmental stage.