In an attempt to ascertain that the phytochrome chromoproteins do indeed constitute the photoreceptor involved in light-harvesting-II-apoprotein-transcript accumulation and the synchronization of the endogenous circadian clock [Tavladoraki, P., Kloppstech, K. & Argyroudi-Akoyunoglou, J. H. (1989) Plant Physiol. (Bethesda) 90, 665-672], we studied the action spectrum of Cab transcription in 9-10-day-old etiolated Phaseolus vulgaris leaves. Transcript accumulation was detected by hybridization experiments or immunoprecipitation of the in-vitro translation products, obtained from isolated leaf poly(A)-rich mRNA species. It was found that the accumulation of the precursor-light-harvesting-II-apoprotein mRNA depends highly on the wavelength of light exposure, the action spectrum of its accumulation being closely correlated with the absorption spectrum of the red-absorbing phytochrome form (Pr); the reversal by far-red light of the light-induced Cab-mRNA accumulation depends also on wavelength of light exposure, used prior to far red for activation of transcription; the action spectrum thus obtained resembles the difference absorption spectrum of the purified phytochrome protein (delta A Pr-Pfr). In addition, immunoblot analysis of total lithium-dodecyl-sulphate-solubilized protein extracts of leaves showed that the appearance of the mature light-harvesting-II apoprotein in etiolated leaves previously exposed to a light pulse of equal dose but varying wavelength, closely follows the pattern of its transcript accumulation and the action spectrum of protochlorophyllide photoconversion. Thus, translation of the protein in vivo, as monitored by the steady level of the protein appearing in Western blots, is closely coordinated with the accumulation of its transcript, the action spectrum of both processes closely resembling the absorption spectrum of phytochrome. The results suggest that phytochrome is involved in photoreception mediating Cab-gene transcription in 9-10-day-old etiolated bean leaves and that Cab transcription, appearance of the mature protein and stabilization of the apoprotein in thylakoids by the chlorophyll formed during the light pulse are in close coordination.