Three classes of human interferons (IFNs) have been defined on the basis of their immunological properties: the 'Le' or 'alpha' IFN, mainly derived from leukocyte or lymphoblastoid cells; the 'F' or 'beta' IFN, mainly derived from fibroblast cultures; and the 'T', 'immune' or 'gamma' IFN, mainly derived from mitogen- or antigen-stimulated lymphoid cells. Whereas several individual species of Le IFN have been purified to homogeneity, it is generally considered that F IFN represents a single protein. Thus current efforts to clone human fibroblast IFN mRNA sequences are based on the observation that F IFN mRNA sediments in sucrose gradients as a single RNA species of size corresponding to 12-14 S (refs 7-10). We show here, using gel electrohporesis of mRNA, that two populations of translationally active human fibroblast IFN mRNA molecules exist--an abundant '14 S' species and a scarce '11 S' species. Microinjection of either species of mRNA into Xenopus oocytes leads to the synthesis of biologically active F-type human IFN. These data agree with and complement recent RNA hybridization studies of Weissenbach et al.

Changes occurring in several chloroplast components during Norflurazon-induced photobleaching of Pisum sativum seedlings were investigated. mRNA steady state levels of the chlorophyll a/b-binding protein of photosystem II, ferredoxin I, the small and large subunits of ribulose 1,5-bisphosphate carboxylase, and pEA214 and pEA207, two other lightresponsive genes, were determined during chlorophyll photooxidation. Relative transcription rates were assayed in isolated nuclei. The results illustrate a complex set of interactions regulating expression of the nuclear and chloroplast genomes. Photobleaching was found to affect the expression of the various genes in different ways. While transcript levels of the chlorophyll a/b-binding protein decreased by more than 80% under photooxidative light conditions in carotenoid-deficient peas, levels of ferredoxin, the small and large subunits of ribulose 1,5-bisphosphate carboxylase, and pEA214 mRNAs were reduced by less than 50%. pEA207 mRNA levels, on the other hand, were resistant to the effects of photobleaching. Analyses of chlorophylls a and b and the chlorophyll al b-binding protein suggest that accumulation of the protein and its mRNA are coordinated with chlorophyll abundance at several steps. In addition to post-transcriptional regulation at the level of mRNA and protein stability, there may exist coordination at the transcriptional stage.Since most (>80%) chloroplast proteins are nuclear encoded (14), the nuclear and chloroplast genomes must coordinate gene expression to bring about normal development of a green plant. The messenger RNAs of the nuclear-encoded chloroplast proteins are translated on cystosolic ribosomes and transported into the plastid where they are incorporated into the photosynthetic machinery. The nuclear-chloroplast interaction can be studied both during greening ofetiolated tissue and by the use ofmutants lacking Chl. In addition, one can study plants that are blocked in carotenoid biosynthesis either because of mutations or due to growth in the presence of herbicides such as Norflurazon [NF4; 4-chloro-5-(methylamino)-2-(a,a,a-trifluoro-m-tolyl)-3(2H)-pyridazinone] (7). In the absence of protecting carotenoids in these plants, high light intensities cause "bleaching" of the green parts(1, 19).

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The effects of high light stress on chloroplast ultrastructure and protein and mRNA composition were investigated in carotenoid-deficient peas (Pisum sativum, L.). In low light, the thylakoid membrane polypeptide pattem was altered, with several prominent chlorophyll-binding proteins present in diminished amounts. This change was found to be reflected in the ultrastructural organization of intemal chloroplast membranes. In contrast to the normal grana stacking found in the controls, carotenoid-deficient plastids contained long, unstacked lamellae. Exposure to photooxidative light that resulted in destruction of >70% of chlorophyll did not lead to changes in total RNA and total cellular protein pattems. This treatment did lead to gross alterations in the chloroplast structure. Within 24 hours the plastid was seen as a swollen vesicle with only a few membrane remnants still present. Accumulation of five plastid-encoded mRNAs encoding a diverse array of photosynthetic proteins was found to be affected in different ways. While psaA mRNA was rapidly reduced by more than 75%, levels of psbF/E and atpB/E were reduced by 50%. psbA and petA mRNAs, on the other hand, appeared to be more resistant to photobleaching and remained relatively unchanged during 24 hours of high fluence-rate light treatment.Long-term exposure of a green plant to strong light can result in injury to the photosynthetic apparatus, the damage being caused in large part as a consequence of light-and oxygen-dependent destruction of the photosynthetic pigments. This bleaching is an extemal symptom of the intracellular photooxidative damage occurring within the chloroplast. In higher plants damage caused as a consequence of Chl photooxidation has been investigated in several plant species (3, 5, 7, 11-13, 18, 19) mainly in relation to the study of the effects of chlorosis-inducing herbicides. More recently, experiments with herbicides inhibiting carotenoid biosynthesis have provided a useful system to study this process with respect both to plastid damage and to its consequences for nuclear gene expression (26).We have previously characterized some of the damaging effects of photooxidative light on pea seedlings (Pisum sati-'This is Carnegie