Transcriptional organization and possible function of mustard plastid DNA regions expressed in vivo

Rapid-cycling populations of brassica

Rapid-cycling populations of six economically important species in the genus Brassica have unusual potential for resolving many problems in plant biology and for use in education. Rapid-cycling brassicas can produce up to ten generations of seed per year and serve as models for research in genetics, host-parasite relations, molecular biology, cell biology, plant biochemistry, population biology, and plant breeding. Brassicas are a highly diverse group of crop plants that have great economic value as vegetables and as sources of condiment mustard, edible and industrial oil, animal fodder, and green manure. These plants can also be used in the classroom as convenient, rapidly responding, living plant materials for "hands on" learning at all levels of our educational system.

Proteolytic removal of the C-terminal transmembrane region of cytochrome f during extraction from turnip and charlock leaves generates a water-soluble monomeric form of the protein

Water-soluble, monomeric cytochrome f purified from leaves of turnip (Brassica rapa) and charlock (Sinapis arvensis) is approximately 3 kDa smaller than the protein in chloroplast thylakoid membranes determined by SDS/PAGE. Sequencing the N-terminal and C-terminal regions of the monomeric protein, by automated Edman degradation and carboxypeptidase P digestion, suggested the loss of 33 amino acid residues at the C-terminus by comparison to sequences of cytochrome f from other higher plants. This was confirmed by the isolation and nucleotide sequencing of the turnip petA gene and by determination of the molecular mass of the monomeric turnip protein by electrospray mass spectrometry. The turnip petA gene encodes a protein of 320 amino acid residues consisting of a presequence of 35 amino acid residues and a mature protein of 285 amino acid residues. The molecular mass of the monomeric turnip protein was 28,160.2 +/- 5.4 Da, indicating cleavage after Gln252 of the mature protein. Electrospray mass spectrometry of the monomeric charlock protein indicated the presence of two main forms with molecular masses of 28,135.1 +/- 5.5 Da and 27,750.7 +/- 4.3 Da corresponding to cleavage after Gln252 and Leu249, respectively. Cleavage in this region of the cytochrome f polypeptide during extraction with butanone removes the single transmembrane span of the protein and liberates the water-soluble globular domain of cytochrome f.

Cytochrome f: Structure, function and biosynthesis

Cytochrome f is an intrinsic membrane component of the cytochrome bf complex, transferring electrons from the Rieske FeS protein to plastocyanin in the thylakoid lumen. The protein is held in the thylakoid membrane by a single transmembrane span located near its C-terminus with a globular hydrophilic domain extending into the lumen. The globular domain of the turnip protein has recently been crystallised, offering the prospect of a detailed three-dimensional structure. Reaction with plastocyanin involves localised positive charges on cytochrome f interacting with the acidic patch on plastocyanin and electron transfer via the surface-exposed tyrosine residue (Tyr83) of plastocyanin. Apocytochrome f is encoded in the chloroplast genome and is synthesised with an N-terminal presequence which targets the protein to the thylakoid membrane. The synthesis of cytochrome f is coordinated with the synthesis of the other subunits of the cytochrome bf complex.

Structure and expression of a split chloroplast gene from mustard (Sinapis alba): ribosomal protein gene rps16 reveals unusual transcriptional features and complex RNA maturation

The mustard chloroplast gene rps16 is split by an 887 bp group II (or III) intron. Three RNA 5' ends upstream of the rps16 coding region define both the transcription start site and two RNA processing sites. The DNA region preceding the start site contains a procaryotic-type "-10" promoter element, but not a typical "-35" element. One single RNA 3' end has been detected downstream from the rps16 coding region, but it is not in close proximity to any inverted repeat that might serve as a termination signal. Northern analysis has revealed several rps16 transcripts ranging in size from 1.6 kb to 0.5 kb. During seedling development, transcript levels show an initial increase and then remain constant without much difference between seedlings grown under light or in the dark.

Six chloroplast genes (ndhA-F) homologous to human mitochondrial genes encoding components of the respiratory chain NADH dehydrogenase are actively expressed: determination of the splice sites in ndhA and ndhB pre-mRNAs

Sequences (ndhA-F) homologous to human mitochondrial genes for components of the respiratory chain NADH dehydrogenase have been found in the chloroplast DNA of tobacco. The ndhA, D, E and F sequences corresponding to the mitochondrial URF1, 4, 4L and 5 are located in the small single copy region, the ndhB sequence corresponding to URF2 in the inverted repeat and the ndhC sequence corresponding to URF3 in the large single copy region of the chloroplast DNA. Northern blot hybridization revealed that all six ndh sequences are actively expressed in the chloroplasts. The ndhA and ndhB sequences contain single introns and the splice sites of their pre-mRNAs were determined by reverse transcription analysis. These findings suggest that potential components of an NADH dehydrogenase are synthesized in the chloroplasts.

Temporal and light control of plastid transcript levels for proteins involved in photosynthesis during mustard (Sinapis alba L.) seedling development

Transcript levels of four plastid genes encoding constituents of the photosynthetic apparatus were assessed in cotyledons of developing mustard (Sinapis alba L.) seedlings. These genes, encoding the P700 apoproteins of photosystem I, the alpha subunit of the extrinsic CF1 moiety of the plastid ATP synthase complex, and the cytochromes f and b 6, have been localized and mapped previously on mustard chloroplast DNA (G. Dietrich and G. Link, 1985 Curr. Genet. 9, 683-692). Dot blot and Northern hybridization analysis provides evidence that in dark-grown seedlings transcript levels of all four genes rise between 30 h and 72 h after sowing and thereafter fall again, pointing to the existence of an endogenous, light-independent, developmental program. In light-grown seedlings, an additional enhancement of transcript levels beyond, the dark values becomes noticeable at approx. 30-36 h and then continues throughout the subsequent "light-responsive" phase until 96 h after sowing. This is consistent with a photoregulated modulation mechanism operating once "competence" has been reached. Enhanced transcript accumulation occurs following continuous illumination by either white light or (photosynthetically inefficient) far-red light thought to operate mainly through phytochrome. However, the degree of light enhancement for the transcript specifying the P700 apoprotiens is higher with white light than with far-red light, implying involvement of additional photoreceptor(s) mediating this response. In addition to the endogenous gross regulation and light enhancement, a fine regulation of transcript levels seems to operate, as indicated by temporal variations of two related transcripts originating from the cytochrome-f gene region. The observed developmental and photocontrolled changes in specific transcript levels for photosynthesis proteins are only reflected in part by changes in total RNA content and do not appear due to light-dark differences in plastid-DNA copy number during mustard seedling development.

Functional in vivo verification in E. coli of promoter activities from the rDNA/tDNA(Val)(GAC) leader region of Zea mays chloroplasts

Restriction fragments containing upstream sequences of the rRNA operon from Zea mays chloroplasts were tested for promoter activity in vivo by insertion into an E. coli promoter-probe vector. The expression of this vector's reporter gene, which codes for alkaline phosphatase, was stimulated more than 1,500-fold upon linkage with the chloroplast rRNA promoter. Site specific mutagenesis of the invariant T of the -10 sequence of this promoter reduced the expression of the reporter gene to 2% of the wild type. This indicates that the chloroplast rRNA promoter, which directs transcriptional initiation 117 bp upstream of the 16S rRNA gene, is also active in the bacterial system. A restriction fragment further upstream containing the gene for tRNA(Val) (GAC) also showed strong promoter activity (29% as compared with the rRNA promoter). This promoter activity probably reflects the chloroplast promoter directing the synthesis of the tRNA(Val) (GAC) primary transcript. Surprisingly, this restriction fragment also displayed promoter activity (13% compared with the rRNA promoter) in reverse orientation.

Transcript levels of two adjacent chloroplast genes during mustard (Sinapis alba L.) seedling development are under differential temporal and light control

Transcript levels of two plastid genes were investigated during early seedling development of mustard (Sinapis alba L.) until 96 h after sowing. The two genes, which are closely linked and have the same polarity, are the psbA gene encoding the Mr 32-35000 herbicide-binding QB-protein of photosystem II and the trnK gene encoding plastid tRNA(Lys) (UUU) and potentially an intron-derived maturase-related protein. By using Northern and dot blot hybridization techniques with sensitive RNA probes, the 1.2 kb psbA transcript was found to be present in low amounts during the initial phase of seed germination. Thereafter, it increases in concentration both in light- and dark-grown seedlings until approximately 48 h after sowing. A further increase in psbA transcript concentration during the subsequent phase until 96 h was observed in light-grown, but not in dark-grown seedlings. The 2.8 kb trnK transcript is one to two orders of magnitude less abundant than the psbA transcript throughout the time period investigated. The concentration of this transcript is light-independent and shows a transient peak level at around 48 h, i.e. at the onset of light-enhanced accumulation of the psbA transcript.

The chloroplast tRNALys(UUU) gene from mustard (Sinapis alba) contains a class II intron potentially coding for a maturase-related polypeptide

The trnK gene endocing the tRNALys(UUU) has been located on mustard (Sinapis alba) chloroplast DNA, 263 bp upstream of the psbA gene on the same strand. The nucleotide sequence of the trnK gene and its flanking regions as well as the putative transcription start and termination sites are shown. The 5' end of the transcript lies 121 bp upstream of the 5' tRNA coding region and is preceded by procaryotic-type "-10" and "-35" sequence elements, while the 3' end maps 2.77 kb downstream to a DNA region with possible stemloop secondary structure. The anticodon loop of the tRNALys is interrupted by a 2,574 bp intron containing a long open reading frame, which codes for 524 amino acids. Based on conserved stem and loop structures, this intron has characteristic features of a class II intron. A region near the carboxyl terminus of the derived polypeptide appears structurally related to maturases.