Plant A-type cyclins

Although the basic mechanisms which control the progression through the cel l cycle appear to be conserved in all higher eukaryotes, the unique feature s of the plant developmental programme must be somehow reflected in a plant -specific regulation of the factors which control cell division. In the pas t few years, considerable progress has been achieved in identifying the maj or components of the cell cycle machinery in plants, especially the cyclin- dependent kinases (CDKs) and their regulatory subunits, the cyclins. The qu estion of how these components direct expression of specific genes at speci fic stages of the cell cycle, and how they are themselves regulated, consti tutes a challenge for the present and for the years to come. This review su mmarizes our current knowledge of a particular class of plant cyclins, the A-type cyclins, which can be further subdivided into three structural group s. The putative functions of these A-type cyclins are discussed in relation to the presence of remarkable motifs in their amino acid sequences, and to their specific transcriptional regulation, protein amount and subcellular localization.