G2-AND EARLY-M-SPECIFIC EXPRESSION OF THE NTCYC1 CYCLIN GENE IN NICOTIANA-TABACUM CELLS

We have previously reported the isolation of a cDNA encoding a mitotic cyclin, NTCYC1, from a tobacco cell suspension library. Here we descr ibe the expression patterns of NTCYC1 and pf Ntsuc1; a suc1 plant homo logue, in synchronized tobacco cell suspensions. Furthermore, the expr ession pattern of this cyclin is compared to that of Ntcdc2-1, a Nicot iana tabacum homologue of cdc2. While no NTCYC1 transcript was detecte d in cells synchronized in the G1 and S phases, NTCYC1 expression was observed in late G2 and early M phases, disappearing in the G1' of a n ew cell cycle. On the other hand, Ntsuc1 and Ntcdc2-1 exhibited a cons titutive expression during the cell cycle. A functional analysis perfo rmed by microinjecting NTCYC1 mRNA into immature Xenopus oocytes, indi cates that NTCYC1 could participate in the control of the G2/M transit ion in plant cells. Subsequently NTCYC1 expression was used to assess the status of mesophyll cells in expanded leaves of N. tabacum. Depend ing on leaf position along the shoot axis, a large population of mesop hyll cells appeared with a 4C DNA content, suggesting a G2 arrest. It was found that leaves with such a population also contained high level s of NTCYC1 transcripts. With respect to these results concerning a na turally occurring G2-arrested cell population, the regulation of NTCYC 1 expression in planta is discussed.