Mechanisms of discrimation of the prolatin physiologic action on the liver at the receptor level of hepatocytes and cholangiocytes

The role ofprolactin receptor isoforms in discrimination ofprolactin effects on liver is summarized. The necessity of studying of parameters ofprolactin receptor expression in differentially isolated different types of liver cells is demonstrated. The review is illustrated by data on different regulation by sex hormones and obstructive cholestasis of expression of prolactin receptor isoforms in rat hepatocytes and epithelial cells of bile ducts. The mathematical model is introduced for estimation of intensity of prolactin-induced signal cascades on the basis of experimentally measured parameters of prolactin receptor expression. Some further steps of the investigation of prolactin effects discrimination at the level receptor unit are suggested.     

Steps in the acquisition of photosynthetic competence by plastids of maize

Coupling factor particles are associated with membranes of maize etioplasts (Lockshin et al., 1971. Biochim. Biophys. Acta 226: 366-382). In addition, several, but not all, of the polypeptides found in the photosynthetic lamellae of maize chloroplasts are present in etioplasts.     

Acid to base phosphorylation and membrane integrity in plastids of greening maize

Fully or partially green chloroplast fragments from maize are capable of synthesizing ATP upon transfer from an acidic to an alkaline environment. It has not been possible to demonstrate this activity in membranes of maize etioplasts. Evidence is presented which shows the development of the capacity to perform acid to base phosphorylation can occur without parallel chlorophyll formation. Acid to base phosphorylation capacity can develop in darkness after brief illumination.

The acid to base capacity seems to have at least two prerequisites: (a) the presence of osmotically responsive internal spaces; and (b) a lamellar component which can be removed by treatment with EDTA. Etioplast fragments possess the EDTA-removable component, but they cannot respond osmotically to changes in the sucrose concentration of their environment. It is suggested that the etioplast's failure to catalyze acid to base phosphorylation is at least in part related to its lack of capacity to exhibit osmotic responsiveness.     

Zea mays chloroplast ribosomal RNA genes are part of a 22,000 base pair inverted repeat

Zea mays chloroplast rDNA exists in two identical units. Each unit contains one sequence for the 16, 23 and 5S rRNAs in the order given. The 16 and 23S sequences in each unit are separated by a 2100 base pair (bp) spacer. The DNA sequence for 5S RNA is closely linked to that for the 23S RNA. Within the above unit, the three RNAs are transcribed from a single DNA strand. The two rDNA units on the circular chloroplast DNA molecule are separated from each other by 18,500 bp in one direction and by 106,100 bp in the other direction. The two rDNA units have an inverted orientation with respect to each other. Each rDNA unit is part of a 22,000 bp sequence which is repeated with inverted orientation.     

DNA sequences for the Zea mays tRNA genes tV-UAC and tS-UGA: tV-UAC contains a large intron

Summary The chloroplast genome contains genes for a large and probably complete set of tRNAs. These genes are unique in sharing attributes of both nuclear and bacterial tRNA genes. Two chloroplast tRNA genes from Zea mays are described here. tV-UAC, encoding a valine tRNA with the anticodon UAC, contains a 603 bp intron and is highly homologous, both in coding regions and in the intron, to the analogous gene from tobacco described by Deno et al. (Nucleic Acids Res 10:7511–7520, 1982). It is located near the gene for the beta and epsilon subunits of the CF₁ complex. (Krebbers et al.: Nucleic Acids Res 10:4985–5002, 1982). The gene tS-UGA, encoding a serine tRNA with the anticodon UGA, is located 41 kbp 3 to tV-UAC. Both genes contain promoter-like sequences in their 5 flanking regions.     

Decreased ribulose-1,5-bisphosphate carboxylase-oxygenase in transgenic tobacco transformed with “antisense” rbcS

The effect of nitrogen supply during growth on the contribution of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco; EC 4.1.1.39) to the control of photosynthesis was examined in tobacco (Nicotiana tabacum L.). Transgenic plants transformed with antisense rbcS to produce a series of plants with a progressive decrease in the amount of Rubisco were used to allow the calculation of the flux-control coefficient of Rubisco for photosynthesis (C_R). Several points emerged from the data: (i) The strength of Rubisco control of photosynthesis, as measured by C_R, was altered by changes in the short-term environmental conditions. Generally, C_R was increased in conditions of increased irradiance or decreased CO₂. (ii) The amount of Rubisco in wild-type plants was reduced as the nitrogen supply during growth was reduced and this was associated with an increase in C_R. This implied that there was a specific reduction in the amount of Rubisco compared with other components of the photosynthetic machinery. (iii) Plants grown with low nitrogen and which had genetically reduced levels of Rubisco had a higher chlorophyll content and a lower chlorophyll a/b ratio than wild-type plants. This indicated that the nitrogen made available by genetically reducing the amount of Rubisco had been re-allocated to other cellular components including light-harvesting and electron-transport proteins. It is argued that there is a luxury additional investment of nitrogen into Rubisco in tobacco plants grown in high nitrogen, and that Rubisco can also be considered a nitrogen-store, all be it one where the opportunity cost of the nitrogen storage is higher than in a non-functional storage protein (i.e. it allows for a slightly higher water-use efficiency and for photosynthesis to respond to temporarily high irradiance).Abbreviations C_R flux control coefficient of Rubisco for photosynthesis - rbcS gene for the Rubisco small subunit - Rubisco ribulose-1,5-bisphosphate carboxylase-oxygenase W.P. Quick is grateful to Professor D.T. Clarkson (Department of Agricultural Sciences, University of Bristol, Long Ashton, UK) for pointing out the connection between stomatal conductance and nutrient availability. This work was supported by the Deutsche Forschungsgemeinschaft.