The comparative analysis of a large number of plant cyclins of the A/B family has recently revealed that plants possess two distinct B-type groups and three distinct A-type groups of cyclins. Despite earlier uncertainties, this large-scale comparative analysis has allowed an unequivocal definition of plant cyclins into either A or B classes. We present here the most important results obtained in this study, and extend them to the case of plant D-type cyclins, in which three groups are identified. For each of the plant cyclin groups, consensus sequences have been established and a new, rational, plant-wide naming system is proposed in accordance with the guidelines of the Commission on Plant Gene Nomenclature. This nomenclature is based on the animal system indicating cyclin classes by an upper-case roman letter, and distinct groups within these classes by an arabic numeral suffix. The naming of plant cyclin classes is chosen to indicate homology to their closest animal class. The revised nomenclature of all described plant cyclins is presented, with their classification into groups CycA1, CycA2, CycA3, CycB1, CycB2, CycD1, CycD2 and CycD3.

Cyclin-dependent protein kinases (CDKs) play key roles in regulating the eukaryotic cell cycle. We have analyzed the expression of four rice (Oryza sativa) CDK genes, cdc2Os1, cdc2Os2, cdc2Os3, and R2, by in situ hybridization of sections of root apices. Transcripts of cdc2Os1, cdc2Os2, and R2 were detected uniformly in the dividing region of the root apex. cdc2Os1 and cdc2Os2 were also expressed in differentiated cells such as those in the sclerenchyma, pericycle, and parenchyma of the central cylinder. By contrast, signals corresponding to transcripts of cdc2Os3 were distributed only in patches in the dividing region. Counterstaining of sections with 4,6-diamidino-2-phenylindole and double-target in situ hybridization with a probe for histone H4 transcripts revealed that cdc2Os3 transcripts were abundant from the G 2 to the M phase, but were less abundant or absent during the S phase. The levels of the Cdc2Os3 protein and its associated histone H1-kinase activity were reduced by treatment of cultured cells with hydroxyurea, which blocks cycling cells at the onset of the S phase. Our results suggest that domains other than the conserved amino acid sequence (the PSTAIRE motif) have important roles in the function of non-PSTAIRE CDKs in distinct cell-cycle phases.

The 7-methylguanylic acid residue confronting the 5'-terminal nucleotide of mRNA through two pyrophosphate linkages was completely removed by tobacco pyrophosphatase from mRNAs of cytoplasmic polyhedrosis virus, tobacco mosaic virus (viral RNA), and globin without any scission in the inner part of the RNA chain. Protein sythesis ability in a wheat germ cell-free system was lost after this treatment of all three kinds of mRNA. The initiation complexes for protein synthesis of these three RNAs were not obtained after using tobacco phosphodiesterase-treated mRNA. On incubation of mRNA in a wheat germ extract, the mRNA lacking m7G was quickly degraded from the 5' terminus in an exonucleolytic way, whereas the intact mRNA remained stable. These results show that one of the confronting nucleotide structure's functions is to stabilize the mRNA, to prevent its degradation.

SummarySynchronously dividing cell cultures of Catharanthus roseus were used to isolate cDNAs for two mitotic cyclins, named CYS and CYM. The deduced protein sequence of CYS is similar to that of A-type cyclins, and CYM belongs to the group of B-type cyclins. In a fashion similar to the pattern of expression seen for A-type and B-type cyclins in mammalian cells, CYS is expressed before CYM in C. roseus cells during the cell cycle. CYS mRNA accumulated at the onset of S phase and disappeared early in the G2 phase, whereas CYM mRNA was detected in the G2 and M phases of the cell cycle. Tobacco homologs of the two genes showed similar cell-cycle dependent expression patterns in synchronous cultures of tobacco BY2 cells. In both systems, CYS was expressed much earlier in the cell cycle than most other plant A-type cyclins, and hence CYS along with the soybean cyclGm can be classified into a distinct subclass. The activities of CYMand CYS promoters during the cell cycle were analyzed in stably transformed tobacco BY2 cells. Cyclin promoter sequences of 0.5 kb could confer the typical cell-cycle-dependent expression to the ~-glucuronidase (GUS) reporter gene: the CYS Received 15 July 1996; revised 20 December 1996; accepted 17 January 1997. *For correspondence (fax +81 3 3814 1728; e-mail masakito@uts2.s.u-tokyo.ac.jp). tPresent address: Laboratoire de Biologie Generale, CNRS Universite Louis Pasteur, 12 Rue de I'Universite, 67000 Strasbourg, France. tPresent address: Botanical Gardens, Faculty of Science, University of Tokyo, Hakusan, Tokyo, 112 Japan.promoter directed S-phase-specific expression, whereas the CYM promoter drove M-phase-specific expression. These results indicate the important role of transcriptional regulation in the oscillations of cyclin mRNA levels during the cell cycle.

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