mRNAs for two ribosomal proteins are preferentially translated in the chloroplast of Chlamydomonas reinhardtii under conditions of reduced protein synthesis

Previous studies have identified a set of highly phosphorylated proteins of 23–25 kDa accumulated during normal embryogenesis of Zea mays L. and which disappear in early germination. They can be induced precociously in embryos by abscisic acid (ABA) treatment. Here the synthesis and accumulation of this group of proteins and their corresponding mRNAs were examined in ABA-deficient viviparous embryos at different developmental stages whether treated or not with ABA, and in water-stressed leaves of both wild-type and viviparous mutants.During embryogenesis and precocious germination of viviparous embryos the pattern of expression of the 23–25 kDa proteins and mRNAs closely resembles that found in non-mutant embryo development. They are also induced in young viviparous embryos by ABA treatment. In contrast, leaves of ABA-deficient mutants fail to accumulate mRNA in water stress, yet do respond to applied ABA. In water-stressed leaves of wild type plants the mRNAs are induced and translated into 4 proteins with a molecular weight and isoelectric point identical to those found in embryos.These results indicate that the 23–25 kDa protein set is a new member of the recently described class or proteins involved in generalized plant ABA responses.The different pattern of expression for the ABA-regulated 23–25 kDa proteins and mRNAs found in embryo and in vegetative tissues of viviparous mutants is discussed.     

Antibiotic Resistance Mutations in the Chloroplast 16s and 23s Rrna Genes of Chlamydomonas Reinhardtii: Correlation of Genetic and Physical Maps of the Chloroplast Genome

Mutants resistant to streptomycin, spectinomycin, neamine/kanamycin and erythromycin define eight genetic loci in a linear linkage group corresponding to about 21 kb of the circular chloroplast genome of Chlamydomonas reinhardtii. With one exception, all of these mutants represent single base-pair changes in conserved regions of the genes encoding the 16S and 23S chloroplast ribosomal RNAs. Streptomycin resistance can result from changes at the bases equivalent to Escherichia coli 13, 523, and 912-915 in the 16S gene, or from mutations in the rps12 gene encoding chloroplast ribosomal protein S12. In the 912-915 region of the 16S gene, three mutations were identified that resulted in different levels of streptomycin resistance in vitro. Although the three regions of the 16S rRNA mutable to streptomycin resistance are widely separated in the primary sequence, studies by other laboratories of RNA secondary structure and protein cross-linking suggest that all three regions are involved in a common ribosomal neighborhood that interacts with ribosomal proteins S4, S5 and S12. Three different changes within a conserved region of the 16S gene, equivalent to E. coli bases 1191-1193, confer varying levels of spectinomycin resistance, while resistance to neamine and kanamycin results from mutations in the 16S gene at bases equivalent to E. coli 1408 and 1409. Five mutations in two genetically distinct erythromycin resistance loci map in the 23S rDNA of C. reinhardtii, at positions equivalent to E. coli 2057-2058 and 2611, corresponding to the rib3 and rib2 loci of yeast mitochondria respectively. Although all five mutants are highly resistant to erythromycin, they differ in levels of cross-resistance to lincomycin and clindamycin. The order and spacing of all these mutations in the physical map are entirely consistent with our genetic map of the same loci and thereby validate the zygote clone method of analysis used to generate this map. These results are discussed in comparison with other published maps of chloroplast genes based on analysis by different methods using many of the same mutants.     

A correlative study of the binding of dexamethasone in hypothalamic blocks in vitro with its ability to inhibit the release of bioactive corticotrophin-releasing factor

Rat hypothalamic blocks incubated in vitro were used to study the characteristics of binding of [3H]dexamethasone and other steroids to cytosolic binding sites. Cytosols prepared following incubation of the tissue with [3H]dexamethasone for 2 h contained specifically bound steroid in amounts that depended upon the concentration of potassium (but not sodium) ions in the extracting buffer. There was an increase in bound [3H]dexamethasone extracted as the potassium ion concentration increased up to 0.1 M, but not beyond. Dexamethasone, when added to hypothalami in vitro caused a biphasic inhibition of bioactive corticotrophin-releasing factor (CRF) release, and the extent of the second phase of inhibition was dose-related. 11-Epicortisol, when added in a 100-fold molar excess over dexamethasone was able to prevent the second phase of inhibition caused by the latter steroid, as in the binding studies it was able to cause a 50% reduction in the binding of [3H]dexamethasone. In the functional studies it was shown that 11-epicortisol was able to "rescue" the tissue from dexamethasone-mediated delayed inhibition of CRF secretion if added to the blocks 30 min (but not later) after the agonistic steroid.     

Hydrogen-peroxide-scavenging systems within pea chloroplasts

The subcellular distribution of ascorbate peroxidase and glutathione reductase (EC 1.6.4.2) in pea leaves was compared with that of organelle markers. Enzyme distribution was found to be similar to that of the chloroplast enzyme NADPH-glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.13). Isolated chloroplasts showed a close correlation between intactness and the percentage of enzyme activity recovered. Chloroplasts of 85% intactness were found to contain a high proportion of leaf dehydroascorbate reductase activity (EC 1.8.5.1), 10% of leaf glutathione and 30% of leaf ascorbate. These results are discussed in relation to the potential role of chloroplast antioxidant systems in plant resistance to environmental and other stress conditions.Abbreviations GSH reduced glutathione - GSSG oxidized glutathione - NADPH-GPD glyceraldehyde-3-phosphate dehydrogenase - SOD superoxide dismutase     

Release of protein from normal and mutant tomato cell walls

The nitrogen content of cell wall preparations from normal tomato (cv Ailsa Craig) fruit remained constant during ripening, whereas salt-soluble protein increased throughout this process. Tomato polygalacturonase released about twice as much protein from the preparations as salts did, with a maximum at the orange stage of development. Polygalacturonase-solubilized protein from the tomato mutant `ripening inhibitor' (rin) was less, and that from the mutant `Never ripe' (Nr) cell walls was more than that from normal wall preparations. Release of protein by fungal cellulase was limited, but was increased by the addition of polygalacturonase from the same source. Salt-solubilized protein contained a range of enzymic activities but these were distributed between fewer multimolecular forms than is the case for whole cell preparations. The results suggest that metabolically active protein, removable by strong salt solutions, cellulase, or polygalacturonase, remains attached to the cell walls of tomato fruit until late in ripening. The unusual amounts of protein attached to the cell walls of mutant fruit appear to be a reflection of the absence of some or all of the isoenzymes of polygalacturonase that are associated with normal ripening.     

24-hour variation in content and release of hypothalamic neuropeptides in the rat

The tissue content of up to eight neuropeptides, viz bombesin (BOM), cholecystokinin (CCK-8), neurotensin (NT), neuropeptide Y (NPY), peptide histidine isoleucine amide (PHI), somatostatin (SRIF), substance P (SP) and vasoactive intestinal polypeptide (VIP), in rat hypothalami removed at various times of the day, was measured using specific radioimmunoassays. There was significant variation in the content of BOM, CCK-8, NT, PHI, SP and VIP across a 24-h period. The levels of BOM, CCK-8 and NT were lowest around the onset of darkness (1900 h) and rose throughout the night to reach a peak around the time of lights on. Hypothalamic content of all eight peptides fell between 0700 h and 1300 h by an average of 45 +/- 4%. Basal release of these peptides, as well as that in the presence of 48 mM potassium (K+), was measured from hypothalami removed between 0700 and 1900 h and incubated in vitro in a CSF-like medium. Basal secretion of NT significantly increased, whilst that of CCK-8 significantly decreased over the same period. There was no significant change in the basal release of the other neuropeptides. The release in the presence of 48 mM K+ of SP decreased significantly during the day, whilst that of VIP significantly increased. There was also a significant change in the stimulated release of BOM, levels falling during the morning and rising again at 1900 h. 48 mM K+ caused a significant increase in the release of SRIF and SP at all times tested. Whilst 48 mM K+ induced a significantly higher release of CCK-8 and NT in the morning, this stimulus was ineffective in the evening. The contrary was true in the case of BOM, NPY and VIP, where a significant stimulation was induced only at 1900 h. The possible implications of these findings are discussed.     

Further characterization of the respiratory deficient dum-1 mutation of Chlamydomonas reinhardtii and its use as a recipient for mitochondrial transformation

Summary The respiratory deficient dum-1 mutant of Chlamydomonas reinhardtii fails to grow in the dark because of a terminal 1.5 kb deletion in the linear 15.8 kb mitochondrial genome, which affects the apocytochrome b (CYB) gene. In contrast to the wild type where only mitochondrial genomes of monomer length are observed, the dum-1 genomes are present as a mixture of monomer and dimer length molecules. The mutant dimers appear to result from head-to-head fusions of two deleted molecules. Furthermore, mitochondrial genomes of dum-1 were also found to be unstable, with the extent of the deletion varying among single cell clones from the original mutant population. The dum-1 mutant also segregates, at a frequency of ca. 4% per generation, lethal minute colonies in which the original deletion now extends at least into the adjacent gene encoding subunit four of NAD dehydrogenase (ND4). We have used the dum-1 mutant as a recipient to demonstrate stable mitochondrial transformation in C. reinhardtii employing the biolistic method. After 4 to 8 weeks dark incubation, a total of 22 respiratory competent colonies were isolated from plates of dum-1 cells bombarded with C. reinhardtii mitochondrial DNA (frequency 7.3 × 10^–7) and a single colony was isolated from plates bombarded with C. smithii mitochondrial DNA (frequency 0.8 × 10^–7). No colonies were seen on control plates (frequency < 0.96 × 10^–9). All transformants grew normally in the dark on acetate media; 22 transformants were homoplasmic for the wild-type mitochondrial genome typical of the C. reinhardtii donor. The single transformant obtained from the C. smithii donor had a recombinant mitochondrial genome containing the donor CYB gene and the diagnostic HpaI and XbaI restriction sites in the gene encoding subunit I of cytochrome oxidase (COI) from the C. reinhardtii recipient. The characteristic deletion fragments of the dum-1 recipient were not detected in any of the transformants.