Expression of a proteasome alpha-type subunit gene during tobacco development and senescence

Proteasomes degrade specific proteins that have been targeted for proteolys is by ubiquitination. In animals and yeast nuclear-localised proteasomes pl ay a role in regulating the cell cycle, and other developmental processes, via control of the levels of regulatory nuclear proteins such as cyclins an d transcription factors. A cDNA, NtPSA1, isolated from tobacco styles was f ound to have high similarity to human and yeast genes, PRCI-human and PRCI- yeast with 63.4% and 51.6% overall identity respectively. These genes are b elieved to encode non-catalytic alpha-type subunits of 26S proteasomes and like NtPSA1 have putative nuclear localisation signals. NtPSA1 RNA was foun d to accumulate to varying levels in different parts of the plant and at di fferent developmental stages. In particular, the level of NtPSA1 RNA was hi gh in young dividing and expanding tissues, and declined during the senesce nce of both leaves and flowers. These results suggest that a role of protea somes in plant nuclei may be to regulate developmental events by controllin g the levels of regulatory proteins in proliferating and developing tissues , rather than to degrade and recycle proteins during senescence.