Is arcA3 a possible mediator in the signal transduction pathway during agonist cell cycle arrest by salicylic acid and UV irradiation?

Progression of BY-2 tobacco cells through the cell cycle was followed after treatments with ultra violet (UV) and salicylic acid (SA) used as a potent inhibitor of the octadecanoid pathway which can mediate response to UV irr adiation. Cells in S phase were more sensitive than G(0)/G(1) or GZ cells t o UV irradiation. Although SA efficiently blocked cells in G(0)/G(1) or G(2 ), it did not block S phase synchronized cells. UV and SA applied simultane ously to cells in G(0)/G(1) delayed the cell cycle progression more than ea ch one separately. Therefore UV irradiation and SA act as agonists to arres t BY-2 cells at cell cycle entry. To further investigate the signalling pathway mediating UV response, we com plemented a W-sensitive Escherichia coli strain with a Nicotiana xanthi cDN A expression library A cDNA (arcA3) whose coding sequence is identical to t he 2,4-D induced arcA cDNA cloned by Ishida et al. (1993) was isolated. We show that arcA3 transcription is induced at cell cycle entry but not direct ly by the 2,4-D treatment, Moreover, arcA3 transcription is induced prior t o the restriction point as shown with the CDK inhibitor roscovitine. The ar cA3 transcription level is increased by UV irradiation but prevented by SA, Indeed, addition of SA prior to UV irradiation blocks the induction of arc A3 transcription. This suggests that arcA3 gene is modulated in both UV and SA responses, the SA effect preceding the UV step. Since arcA3 is 67% simi lar to RACK1 (functional homology), a rat intracellular receptor for protei n kinase C, and possesses identical PKC fixation motifs, it is hypothesised that the arcA3 gene is involved in UV and SA cell cycle arrest.