Plastid-encoded Genes Research Articles

Accumulation of photosynthesis gene transcripts in response to sodium chloride by salt-tolerant alfalfa cells

A cell line from diploid alfalfa (Medicago sativa L.), HG2-N1, which is stably tolerant to 1% (0.171 M) NaCl in the medium and noticeably more green than the parent line (HG2), was investigated to determine if increased chlorophyll accumulation was related to chloroplast development toward greater photosynthetic activity and increases in chloroplast-gene expression in this cell line. The ctDNA (chloroplast-DNA) copy number in HG2-N1 was found to be increased by 50% from that of the salt-sensitive parent HG2 line. RNA accumulation, as detected by hybridization to a nuclear actin gene, appeared comparable in HG2 and HG2-N1. In contrast, mRNA levels of the plastid-encoded psbA gene were increased in the salt-tolerant HG2-N1 grown on normal medium. The mRNA levels of HG2-N1 were further increased in response to added NaCl in the medium. The mRNA levels from other chloroplast genes necessary for photosynthesis (psbD, psaB, atpB, rbcL), as well as from several nuclear genes (pCab4, pCab1, rbcS) encoding polypeptides participating in photosynthesis, also increased in the salt-tolerant HG2-N1 when it was grown in the presence of NaCl whereas actin-mRNA and chloroplast-rRNA levels remained comparable under growth conditions ± NaCl. Several independently isolated salt-tolerant cell lines showed a similar response to salt, as monitored by accumulation of rbcL mRNA, indicating that transcript accumulation for photosynthesis genes is correlated with the salt response in these salt-tolerant alfalfa cell lines.

Localization of ?-phycoerythrin to the thylakoid lumen of Cryptomonas ? does not involve a signal peptide

Recent investigations have shown that, in cryptomonads, the phycobiliproteins are located within the thylakoid lumen rather than on the stromal side of this membrane as found in cyanobacteria and red algae. To examine possible mechanisms involved in targeting this protein to the thylakoid lumen, the plastid-encoded cpeB gene from Cryptomonas phi was sequenced. This gene encodes an open reading frame of 177 amino acids that is highly similar to known beta-phycoerythrin proteins. cpeB is expressed as a monocistronic transcript of approximately 680 bases. The genes for the alpha subunits of phycoerythrin are not co-transcribed with cpeB nor located anywhere near it. No evidence of amino- or carboxy-terminal extensions or interior modifications involved in directing the Cryptomonas beta-phycoerythrin into the lumen of the thylakoid could be detected. These data suggest that a novel mechanism may be involved in directing cryptomonad biliproteins to the thylakoid lumen.

Structure and biogenesis of Chlamydomonas reinhardtii photosystem I.

The photosystem I complex of the green alga Chlamydomonas reinhardtii was isolated and fractionated into its two subcomplex components: the core complex (CC I), which contained the reaction center (P-700) and had four polypeptide subunits, and the light-harvesting complex (LHC I) which contained four polypeptides of about 22, 25, 26 and 27 kDa. The 22-kDa apoprotein was isolated as a chlorophyll a and b binding protein. In the isolated photosystem I holocomplex, about ten copies of the 22-kDa LHC I apoprotein are present for each CC I unit. The 22-kDa polypeptide as well as the other three polypeptides of this complex and the subunit II of CC I are translated on 80S cytoplasmic ribosomes, and therefore are coded in the nucleus. During the greening process of the Chlamydomonas reinhardtii y-1 mutant the 22-kDa LHC I polypeptide, which cross-reacts with polyclonal antibodies raised against the Lemna gibba 20-kDa LHC I apoprotein, accumulates in thylakoids at a late stage of their development, and about 2-3 h after the LHC II and CC I subunit II polypeptides have accumulated. Accumulation of the 22-kDa protein during greening is inhibited by cycloheximide but not by chloramphenicol.

Molecular evolution and nucleotide sequences of the maize plastid genes for the alpha subunit of CF1 (atpA) and the proteolipid subunit of CF0 (atpH).

The nucleotide sequences of the maize plastid genes for the alpha subunit of CF1 (atpA) and the proteolipid subunit of CF0 (atpH) are presented. The evolution of these genes among higher plants is characterized by a transition mutation bias of about 2:1 and by rates of synonymous and nonsynonymous substitution which are much lower than similar rates for genes from other sources. This is consistent with the notion that the plastid genome is evolving conservatively in primary sequence. Yet, the mode and tempo of sequence evolution of these and other plastid-encoded coupling factor genes are not the same. In particular, higher rates of nonsynonymous substitution in atpE (the gene for the epsilon subunit of CF1) and higher rates of synonymous substitution in atpH in the dicot vs. monocot lineages of higher plants indicate that these sequences are likely subject to different evolutionary constraints in these two lineages. The 5'- and 3'-transcribed flanking regions of atpA and atpH from maize, wheat and tobacco are conserved in size, but contain few putative regulatory elements which are conserved either in their spatial arrangement or sequence complexity. However, these regions likely contain variable numbers of "species-specific" regulatory elements. The present studies thus suggest that the plastid genome is not a passive participant in an evolutionary process governed by a more rapidly changing, readily adaptive, nuclear compartment, but that novel strategies for the coordinate expression of genes in the plastid genome may arise through rapid evolution of the flanking sequences of these genes.

Transcriptional and post-transcriptional regulation of chloroplast gene expression in Petunia hybrida

To study the control of differential gene expression during plastid biogenesis in Petunia hybrida, we have investigated the in vivo translation and transcription of the rbc L gene, coding for the large subunit of ribulose bisphosphate carboxylase (LSU), and the psa A gene, coding for P700 chlorophyll-a apoprotein (AP700). Differential expression of these plastid-encoded genes was studied in two developmentally different plastid systems, proplastid-like organelles from the green cell suspension AK2401 and mature chloroplasts from green leaves. In vivo translation of rbc L and psa A transcripts was analysed using specific antibodies. Specific transcript levels were analysed using internal fragments of the rbc L and psa A genes. A standardization procedure was used so that a direct correlation could be made between the amount of products and gene copy number. In Petunia hybrida the amount of LSU polypeptides present in both plastid types does not correspond to the amount of specific mRNA for the gene. Although the rbc L transcripts are present in both plastid types, the LSU protein is only present in green leaf plastids and not in cell culture plastids. In vitro translation of isolated rbc L transcripts give similar results, thereby suggesting that differences in the primary structure of the transcripts are responsible for the observed discrepancy. In contrast to this, the amount of AP700 polypeptides does correspond to the amount of the psa A transcripts. Therefore, our results indicate that the expression of chloroplast genes during plastid biogenesis takes place on at least two different levels: expression of the rbc L gene is regulated post-transcriptionally while expression of the psa A gene is regulated at the transcriptional level.

Expression of nuclear and plastid genes for photosynthesis-specific proteins during tomato fruit development and ripening

The expression of plastid and nuclear genes coding for photosynthesis-specific proteins has been studied during tomato fruit formation. The steady-state transcript levels for the large (rbcL) and small (rbcS) subunit of RuBPC/Oase, as well as the thylakoid membrane proteins, the 32 kD QB-binding protein of PS II (psbA), the P700 reaction center protein of PS I (psaA) and the chlorophyll a/b-binding protein (cab) vary at different time points during fruit development and ripening. Messenger RNA levels of plastid-encoded photosynthesis-specific genes (rbcL, psbA) are at least several fold higher, relative to respective nuclear-encoded genes (rbcS, cab). The transcript levels for the large and small subunit of RuBPC/Oase are highest in approximately 14-day-old tomato fruits, while the chl a/b-binding protein, the P700 reaction center protein and the 32 kD QB-binding protein reach their maxima in approximately 7-, 14- and 25-day-old tomato fruits, respectively. The inactivation of the photosynthesis-specific genes occurs during the first period of fruit formation. In addition, there is considerable variation in the mRNA levels of these photosynthesis-specific genes in four organs of tomato (leaves, fruits, stems, roots).