In vitro expression of chloroplast genes in lysates of higher plant chloroplasts

Characterization of dynamics of the psbD light-induced transcription in mature wheat chloroplasts

Dynamical aspects of three chloroplast promoters responding to change in light condition were examined in mature chloroplasts of wheat (Triticum aestivum) by in vitro transcription. The wheat psbD/C operon has four distinct promoters, two of which named as D/C-3 and D/C-4 promoters dominantly function in mature chloroplasts to produce the mRNAs encoding D2/CP43 and CP43 alone, respectively. Activity of the D/C-3 promoter in mature chloroplasts was reduced to less than 30% by 24 h dark adaptation and recovered by re-illumination to the original level within 30 to 60 min. The activation of the D/C-3 promoter which requires de novo cytoplasmic protein synthesis was induced by low fluence of light (e.g. 16 microE m(-2) s(-1)), but the extent of activation increased with increasing light fluence. The accumulation of mRNAs from the D/C-3 promoter saturated at 2- to 3-fold higher level within 2 h when the dark-adapted seedlings were transferred to the light at 72 microE m(-2) s(-1), concomitant with the increase in rate of D2 synthesis, suggesting that synthesis of D2 in mature chloroplasts is controlled via the D/C-3 promoter activity in a light-dependent way. Activity of the D/C-4 promoter slightly increased in the dark and decreased in the light. Effect of light on the psbA promoter activity was not observed at all in mature chloroplasts. In vitro transcriptional analysis of the D/C-3 promoter with 5' deletion mutations revealed that at least two cis elements which are located within the sequences of -78 to -47 and -46 to -29 of the transcription initiation site, respectively, act as enhancing elements in the D/C-3 promoter. The light-switching element of the transcription, however, was suggested to be located in the core promoter sequence downstream of the -35 element.

Subcellular Location of O-Acetylserine Sulfhydrylase Isoenzymes in Cell Cultures and Plant Tissues of Datura innoxia Mill

O-Acetylserine sulfhydrylase (OASS; EC 4.2.99.8) catalyzes the formation of L-cysteine from O-acetylserine and inorganic sulfide. Three OASS isoenzymes that differ in molecular mass and subunit structure are present in shoot and root tissues and in cadmium-resistant and cadmium-susceptible cell cultures of Datura innoxia Mill. Different OASS forms predominate in leaves, roots, and suspension-cell cultures. To determine the subcellular location of the OASS isoenzymes, purified mitochondria, chloroplasts, and cytosolic fractions from protoplasts were obtained. The isoenzymes are compartmentalized in D. innoxia cells, with a different isoenzyme predominant in the chloroplast, cytosol, and mitochondria, suggesting that they serve different functions in the plant cell. The chloroplast form is most abundant in green leaves and leaf protoplasts. The cytosolic form is most abundant in roots and cell cultures. A mitochondrial form is abundant in cell cultures, but is a minor form in leaves or roots. Cadmium-tolerant cell cultures contain 1.8 times as much constitutive OASS activity as the wild-type cell line, and 2.9 times more than the cadmium-hypersensitive cell line. This may facilitate rapid production of glutathione and metal-binding phytochelatins when these cultures are exposed to cadmium.

Cis-acting elements and trans-acting factors for accurate translation of chloroplast psbA mRNAs: development of an in vitro translation system from tobacco chloroplasts

Translational regulation is an important step of gene expression in chloroplasts. To analyze biochemical mechanisms of translational regulation unique to higher plant chloroplasts, an in vitro translation system has been developed from tobacco chloroplasts. Conditions for chloroplast extraction and the in vitro translation reaction have been optimized with a tobacco psbA-lacZ fusion mRNA. The in vitro system supports accurate translation of a variety of chloroplasts mRNAs. Using a series of mutant psbA mRNAs, we showed that three elements within the 5'-untranslated region of the mRNA are required for translation. Two of them are complementary to the 3'-terminus of chloroplast 16S rRNA (termed RBS1 and RBS2) and the other is an AU-rich sequence (UAAAUAAA) located between RBS1 and RBS2 and is termed the AU box. mRNA competition experiments using the in vitro translation reaction and gel mobility shift assays revealed the existence of a trans-acting factor(s) for translation and its possible interaction with the AU box. We propose a model for the initiation of psbA translation whereby RBS1 and RBS2 bind cooperatively to the 3'-end of 16S rRNA resulting in looping out of the AU box, which facilitates the interaction of a trans-acting factor(s).

Synthesis and assembly of large subunits into ribulose bisphosphate carboxylase/oxygenase in chloroplast extracts

We have developed a new system for the in vitro synthesis of large subunits and their assembly into ribulose bisphosphate carboxylase oxygenase (Rubisco) holoenzyme in extracts of higher plant chloroplasts. This differs from previously described Rubisco assembly systems because the translation of the large subunits occurs in chloroplast extracts as opposed to isolated intact chloroplasts, and the subsequent assembly of large subunits into holoenzyme is completely dependent upon added small subunits. Amino acid incorporation in this system displayed the characteristics previously reported for chloroplast-based translation systems. Incorporation was sensitive to chloramphenicol or RNase but resistant to cycloheximide, required magnesium, and was stimulated by nucleotides. The primary product of this system was the large subunit of Rubisco. However, several lower molecular weight polypeptides were formed. These were structurally related to the Rubisco large subunit. The initiation inhibitor aurintricarboxylic acid (ATA) decreased the amount of lower molecular weight products accumulated. The accumulation of completed large subunits was only marginally reduced in the presence of ATA. The incorporation of newly synthesized large subunits into Rubisco holoenzyme occurred under conditions previously identified as optimal for the assembly of in organello-synthesized large subunits and required the addition of purified small subunits.

In-vivo and in-vitro synthesis of photosynthetic fructose-1,6-bisphosphatase from pea (Pisum sativum L.)

Etiolated pea (Pisum sativum L. cv. Lincoln) seedlings do not show any capability for the biosynthesis of chloroplast fructose-1,6-bisphosphatase (FBPase), but the rate of biosynthesis of the increases with the pre-illumination time. This light-induced FBPase synthesis appears to be regulated at the transcriptional level, the response of young leaves being greater than that of mature ones. In-vivo labelling experiments demonstrated by immunoprecipitation, followed by sodium dodecyl sulfate electrophoresis and fluorography, the presence of a 49-kilodalton (kDa) band which corresponds to the mature FBPase subunit. In-vitro translation experiments with a wheat-germ synthesizing system and polyadenylated mRNA isolated from illuminated young pea seedlings have demonstrated the appearance of a 59-kDa labelled band corresponding to the precursor of the FBPase basic subunit. When intact pea chloroplasts were added to the above in-vitro incubation mixture, a labelled 49-kDa subunit similar to that of the in-vivo experiments appeared in the organelle under illumination. From these results we can conclude that a 10-kDa transit peptide bound to the translated pea FBPase subunit exists in the cytosol; this transit peptide is lost during passage through the chloroplast envelope, leaving the mature subunit inside the organelle.

In vitro synthesis, assembly and function of a photosynthetic membrane protein

Cell-free translation of Chlamydomonas reinhardtii RNA in the presence of photosynthetic membranes resulted in association of the herbicide binding (Qb) protein with membranes. Incubation of recovered membranes with high salt did not extract the polypeptide from membranes. Tryptic digestion of in vivo labeled membranes or membranes recovered from in vitro translation mixtures showed that Qb had similar orientation. In vitro translation in the presence of chloroplast membranes from cells exposed to high light intensity restored the membrane associated kinase activity lost by photoinhibition. Thus, in vitro synthesis resulted in functional integration of the Qb protein within the photosynthetic membrane.

Proteins homologous to leaf glycoproteins are abundant in stems of dark-grown soybean seedlings. Analysis of proteins and cDNAs

We report here the cloning and sequence analysis of cDNAs for a pair of closely related proteins from soybean (Glycine max [L.] Merr. cv. Williams 82) stems. Both proteins are abundant in soluble extracts of seedling stems but not of roots. One of these proteins (M r=28 kDa) is also foundd in the cell wall fraction of stems and actumulates there when seedlings are exposed to mild water deficit for 48 h. The mRNA for these proteins is most abundant in the stem region which contains dividing cells, less abundant in elongating and mature stem cells, and rare in roots. Using antiserum against the 28 kDa protein, we isolated cDNA clones encoding it and an antigenically related 31 kDa protein. The two cDNAs are 80% homologous in nucleotide and amino acid coding sequence. The predicted proteins have similar hydropathy profiles, and contain putative NH2-terminal signal sequences and a single putative N-linked glycosylation site. The two proteins differ significantly in calculated pI (28 kDa=8.6; 31 kDa=5.8), and the charge difference is demonstrated on two-dimensional gels. The proteins described here may function as somatic storage proteins during early seedling development, and are closely related to glycoproteins which accumulate in vacuoles of paraveinal mesophyll cells of fully expanded soybean leaves when plants are depodded.

Structure and expression of the gene coding for the alpha-subunit of DNA-dependent RNA polymerase from the chloroplast genome of Zea mays

The rpoA gene coding for the alpha-subunit of DNA-dependent RNA polymerase located on the DNA of Zea mays chloroplasts has been characterized with respect to its position on the chloroplast genome and its nucleotide sequence. The amino acid sequence derived for a 39 Kd polypeptide shows strong homology with sequences derived from the rpoA genes of other chloroplast species and with the amino acid sequence of the alpha-subunit from E. coli RNA polymerase. Transcripts of the rpoA gene were identified by Northern hybridization and characterized by S1 mapping using total RNA isolated from maize chloroplasts. Antibodies raised against a synthetic C-terminal heptapeptide show cross reactivity with a 39 Kd polypeptide contained in the stroma fraction of maize chloroplasts. It is concluded that the rpoA gene is a functional gene and that therefore, at least the alpha-subunit of plastidic RNA polymerase, is expressed in chloroplasts.

Protein synthesis by isolated chloroplasts

Isolated chloroplasts show substantial rates of protein synthesis when illuminated. This 'in organello' protein synthesis system has been advantageously utilised to elucidate the coding capacity of chloroplast and the regulation of chloroplast genes. The system is also being used recently to transcribe and translate homologous and heterologous templates. In this mini-review, we attempt to critically ecaluate the available literature and present the current and the prospective lines of research.

Specific in vitro transcription of 16S rRNA gene by pea chloroplast RNA polymerase

A highly purified RNA polymerase preparation from pea chloroplasts has been shown to specifically transcribe the 16S rRNA gene in vitro using the recombinant pCB2-8 DNA as a template. The RNA polymerase has been found to show maximum activity and specificity with pea supercoiled rDNA as a template. At low concentrations of ribonucleoside triphosphates, the RNA polymerase selectively initiates transcription on the 16S rRNA gene. A part of the 16S rRNA gene has been sequenced. The mature 16S rRNA has been found by S1 nuclease analysis to contain sequences starting from GAAGCT. The in vitro synthesized RNA has been found to protect the same nucleotides GAAGCT. In addition, the in vitro synthesized RNA was also found to strongly protect bases starting with TATG located at about 260 bases away from the start site of the mature 16S rRNA.