MOLECULAR CHARACTERIZATION OF A BETA-TYPE PROTEASOME SUBUNIT FROM ARABIDOPSIS-THALIANA COEXPRESSED AT A HIGH-LEVEL WITH AN ALPHA-TYPE PROTEASOME SUBUNIT EARLY IN THE CELL-CYCLE

Proteasomes are the essential components of complexes involved in an e xtralysosomal energy-and ubiquitin-dependent proteolytic pathway. The first alpha-type proteasome subunit in plants has recently been descri bed. In this work, the sequence of the first beta-type proteasome subu nit in plants, isolated from Arabidopsis thaliana cDNA libraries is re ported. The mRNA accumulation of both subunits was analysed and compar ed with those of the ubiquitin and histone mRNAs, in different tissues and during re-initiation of mitotic activity. It is demonstrated that in plants, as in animal cells, the transcripts of both the alpha-type and beta-type proteasome subunits accumulate to high levels during ce ll proliferation, in parallel with mRNAs coding for a ubiquitin fusion protein and several polyubiquitins, but earlier than those coding for histone H4 whose expression is known to be coupled to DNA synthesis. These results suggest that, as in animal cells, proteasomes may be inv olved in the progression of the cell cycle.