No abstract

We have investigated the distribution of transcripts for chloroplast proteins involved in photosynthesis and carbon assimilation in Arabidopsis thaliana. The probes that were used for assessment of transcripts represent the chloroplast genes psbA (encoding the D1 protein of photosystem 11) and rbcL (for the large subunit of RubisCO) as well as the nuclear rbcS gene family for the RubisCO small subunit. With any of these probes we find a tissue-specific distribution of transcripts in mature plants, with the highest concentration in rosette leaves and almost undetectable amounts in roots. The transcripts detected by each of these probes reveal similar spatial distribution and temporal kinetics during seedling development, with maximal levels around 70 h after sowing. They each show higher levels under light-grown as compared to dark-grown conditions, indicating a coarse control that ensures roughly coordinated levels throughout plastid differentiation. Embryonic transcripts are present in much higher concentration within the ground meristem and protoderm than in the procambium, which seems to reflect the different degree of determination towards the subsequent function in photosynthesis. Transcripts were detected in all characteristic stages throughout embryogenesis including the (non-green) proembryo stage, suggesting that chlorophyll accumulation is not essential for expression of these genes at the RNA level. Both processes, hence, appear to be triggered by earlier molecular events that remain to be characterized.

The small subunit of ribulose‐1,5‐bisphosphate carboxylase/oxygenase is encoded by a nuclear multigene family (rbcS genes). We have cloned and characterized three rbcS cDNA sequences from Brassica napus. These cDNA clones all appear to encode the same protein, but they differ in their nucleotide sequence, which has been exploited in the construction of clone‐specific oligonucleotides as selective hybridization probes. By using a nonselective RNA probe, the temporal expression of the entire rbcS gene family was analyzed during seedling development of Brassica napus and of Sinapis alba. In both crucifer species, rbcS transcripts show transient peak levels and then decrease, although to a different degree. Only a moderate (twofold) difference in transcript pool sizes is observed in light‐grown versus dark‐grown seedlings at the time of peak levels, while a much higher light/dark ratio is found in late‐stage seedlings. The oligonucleotide probes reveal three subsets of transcripts which differ in their accumulation kinetics and light/dark ratio. Assessment of the spatial distribution by using in situ hybridization indicates that rbcS transcripts are uniformly localized in cross‐sections of cotyledons from either light‐grown or dark‐grown seedlings, whereas they are undetectable in root sections.

Several genes which are located close together on mustard (Sinapis alba L.) chloroplast DNA have been found to differ in their temporal mode of expression throughout seedling development. One predominant expression program, exemplified by thepsbA gene, is characterized by an early (light-independent) rise in transcript levels, followed by subsequent further accumulation to levels which are much higher in the light than in darkness (development of 'competence' for photocontrol). Other genes located next to thepsb A gene show transient or constitutive modes of expression, with no light-dark difference in transcript levels throughout seedling development. The characteristics of light-responsive expression were shown for the nuclearrbcS gene family inBrassica napus L. andSinapis alba L. cotyledons. The spatial distribution ofrbcS andpsbA transcripts across sections of crucifer cotyledons appeared to be relatively uniform, but restricted to photosynthetically active cells. Finally, assessment of these transcripts in immature seeds and embryos ofCapsella bursa-pastoris has provided in-situ evidence for tissuespecific gene expression during early development.

The Qв-protein is as a hydrophobic integral membrane protein firmly bound in the reaction center complex of photosystem II. A new method was developed to purify the SDS extracted protein using reversed-phase chromatography with two binary linear gradient systems. The identification of the Qв-protein was achieved by its rapidly labeling during photoassimilation of [35S]sulfate and by its reaction with the photoaffinity label azido-[14C]atrazine. Furtherm ore, antisera against the purified Qв-protein reacted with a single peak fraction, the second peak of the chrom atogram , which was identical with the labeled protein peak fraction.