Tobacco cell suspension cultures responded to cytokinins (for instance kinetin) by full chloroplast differentiation. The hormone had the effect of stimulating the appearance of a few prominent plastid proteins. Synthesis of the light-harvesting chlorophyll a/b-binding protein (LHCP) in response to kinetin was noteworthy (Axelos M. et al.: Plant Sci Lett 33:201-212, 1984).Poly(A) +RNAs were prepared from cells grown in the presence of or without added kinetin. Poly(A) + RNA recovery and translation activity were not quantitatively altered by the hormone treatment. In vitro translation of polyadenylated mRNA into precursor polypeptides of LHCP (pLHCP) was quantified by immunoprecipitation and SDS-PAGE fractionation of p L H C P immunoprecipitates: p L H C P -m R N A translating activity was found to be stimulated in parallel to mature LHCP accumulation by kinetin-induced cells.Dot-blot and northern-blot hybridizations of poly(A)+RNA were carried out, using as a probe a pea LHCP-cDNA clone (Broglie R. et al.: Proc Natl Acad Sci USA 78:7304-7308, 1981). A ten-fold increase of the level of pLHCP-encoding sequences was observed in poly(A)+RNA prepared from 9-d kinetin-stimulated cells, compared to control cells. Oligo(dT)-cellulose-excluded RNA fractions exhibited very low hybridization levels, in the same ratios as those obtained with poly(A)+RNA.Thus, the expression of LHCP-gene activity, in response to kinetin addition to tobacco cell suspension cultures, is regulated by the level of pLHCP-encoding mRNA rather than by translational or post-translational controls.