Identification of a Cell Cycle-Related Gene, Cyclin, in Nicotiana tabacum (L.)

Among a11 of the proteins known to be involved in the cell cycle, the CDKs are key components in the control of both the G, to S phase and the G, to M phase transitions in a11 eukaryotes (Jacobs, 1992; Nigg, 1993). The kinase activity of CDKs is regulated by subunits called cyclins, which are involved in the modification of the phosphorylation state, the specificity, and the subcellular location of the enzymes. Although these mechanisms are supposed to occur in a11 eukaryotes, the multiplicity and the role of cyclins within plants are comparatively less well documented. Severa1 plant cyclin genes have nevertheless been characterized in various species such as Arabidopsis (Hemerly et al., 1992) and maize (Renaudin et al., 1994). No related gene has been isolated so far in tobacco; therefore, we focused our attention on the isolation of cyclins from Nicotiana tabacum to study cell-cycle regulation in this model plant (Table I). The 1656-bp fragment isolated from a tobacco cell-suspension cDNA library has been identified as a cyclin cDNA with respect to its sequence similarity with other yeast, animal, and plant cyclins yet characterized. This cDNA sequence, named NTCYCl, contains an open reading frame encoding a protein of 447 amino acids. The predicted primary sequence of the NTCYCl gene product shows regions that are conserved among mitotic cyclins. The C terminus contains a putative cyclin box between the amino acid residues Met’” and Trp354, which is known to be involved in binding to and activation of protein kinases of the CDK family (Nugent et al., 1991). Also, the sequence from Met’” to within the cyclin box is similar to the P-box of cyclin B. This consensus motif is necessary for the activation of the protein phosphatase cdc25 underlying activation of the cyclin B-~34‘~‘’ complex during the GJM phase transition (Zheng and Ruderman, 1993). Additional conserved domains are found in the N-terminal part of the sequence outside the cyclin box. The motif RXXLXXXXN specifies the destruction box well characterized in mitotic cyclins. This acts as a signal for the degradation of cyclin at the end of mitosis (Nugent et al., 1991). The mechanism whereby the NTCYCl cyclin might regda t e kinase activity during cell division in tobacco is not