Leaf development and phytochrome modulate the activation ofpsbD-psbC transcription by high-fluence blue light in barley chloroplasts

An extended AE-rich N-terminal trunk in secreted pineapple cystatin enhances inhibition of fruit bromelain and is posttranslationally removed during ripening

Phytocystatins are potent inhibitors of cysteine proteases and have been shown to participate in senescence, seed and organ biogenesis, and plant defense. However, phytocystatins are generally poor inhibitors of the cysteine protease, bromelain, of pineapple (Ananas comosus). Here, we demonstrated that pineapple cystatin, AcCYS1, inhibited (>95%) stem and fruit bromelain. AcCYS1 is a unique cystatin in that it contains an extended N-terminal trunk (NTT) of 63 residues rich in alanine and glutamate. A signal peptide preceding the NTT is processed in vitro by microsomal membranes giving rise to a 27-kD species. AcCYS1 mRNA was present in roots and leaves but was most abundant in fruit. Using immunofluorescence and immunoelectron microscopy with an AcCYS1-specific antiserum, AcCYS1 was found in the apoplasm. Immunoblot analysis identified a 27-kD protein in fruit, roots, and leaves and a 15-kD species in mature ripe fruit. Ripe fruit extracts proteolytically removed the NTT of 27-kD AcCYS1 in vitro to produce the 15-kD species. Mass spectrometry analysis was used to map the primary cleavage site immediately after a conserved critical glycine-94. The AE-rich NTT was required to inhibit fruit and stem bromelain (>95%), whereas its removal decreased inhibition to 20% (fruit) and 80% (stem) and increased the dissociation equilibrium constant by 1.8-fold as determined by surface plasmon resonance assays. We propose that proteolytic removal of the NTT results in the decrease of the inhibitory potency of AcCYS1 against fruit bromelain during fruit ripening to increase tissue proteolysis, softening, and degradation.

A role of the -35 element in the initiation of transcription at psbA promoter in tobacco plastids

Most plastid promoters recognized by bacteria-like plastid RNA polymerase (PEP) are similar to E. coli sigma(70)-type promoters comprising "-35" and "-10" elements. Among them, psbA promoter is unique in bearing additional elements between the conserved -35 and -10 elements. The psbA promoter activity is differentially maintained in the mature chloroplasts where the activity of most PEP promoters declines. Previously, we identified two types of PEP activities in wheat seedlings [Satoh et al. (1999) Plant J. 18: 407]; PEP present in the mature chloroplasts of the leaf tip (tip-type PEP) can initiate transcription from the -35-destructed psbA promoter, but the -35 element is essential for transcription by PEP present in immature chloroplasts of the leaf base (base-type PEP). To reveal which type of PEP functions in various types of plastids in tobacco, we analyzed the tobacco psbA promoter by means of a transplastomic approach. The promoter core context (-42 to +9) was sufficient for developmental regulation of the psbA promoter activity. The -35 promoter element was important for transcription initiation at the psbA promoter in all types of plastids, including chloroplasts in mature leaves, leucoplasts in roots, etioplasts in etiolated cotyledons. The conclusion is that the PEP bearing a promoter preference, similar to the wheat base-type PEP, functions dominantly in tobacco chloroplasts.

Identification and regulation of high light-induced genes in Chlamydomonas reinhardtii

We have used restriction fragment differential display for isolating genes of the unicellular green alga Chlamydomonas reinhardtii that exhibit elevated expression on exposure of cells to high light. Some of the high light-activated genes were also controlled by CO2 concentration. Genes requiring both elevated light and low CO2 levels for activation encoded both novel polypeptides and those that function in concentrating inorganic carbon (extracellular carbonic anhydrase, low CO2-induced protein, ABC transporter of the MRP subfamily). All the genes in this category were shown to be under the control of Cia5, a protein that regulates the responses of C. reinhardtii to low-CO2 conditions. Genes specifically activated by high light, even under high-CO2 conditions, encoded a 30 kDa chloroplast membrane protein, a serine hydroxymethyltransferase, a nuclease, and two proteins of unknown function. Experiments using DCMU, an inhibitor of photosynthetic electron transport, and mutants devoid of either photosystem I or photosystem II activity, showed aberrant expression of all the genes regulated by both CO2 and high light, suggesting that redox plays a role in controlling their expression. In contrast, there was little effect of DCMU or lesions that block photosynthetic electron transport on the activity of genes that were specifically controlled by high light.

Blue light specific and differential expression of a plastid sigma factor, Sig5 in Arabidopsis thaliana

The transcription of plastid gene psbD is under the control of the BLRP (blue-light-responsive promoter) recognized by plastid-encoded RNA polymerase, in which nuclear-encoded sigma factors play a crucial role in the promoter recognition. We examined the effects of light on mRNA levels of six different SIG genes in Arabidopsis and found that blue light extensively induced the accumulation of SIG5 transcripts, but red light did not. The blue light specificity was not observed in the accumulations of remaining five SIG genes. The blue light dependency of the SIG5 expression well explains the light-dependent behavior of the psbD BLRP.

Phytochrome A mediates blue light and UV-A-dependent chloroplast gene transcription in green leaves

We characterized the photobiology of light-activated chloroplast transcription and transcript abundance in mature primary leaves by using the following two systems: transplastomic promoter-reporter gene fusions in tobacco (Nicotiana tabacum), and phytochrome (phyA, phyB, and hy2) and cryptochrome (cry1) mutants of Arabidopsis. In both dicots, blue light and UV-A radiation were the major signals that activated total chloroplast and psbA, rbcL, and 16S rrn transcription. In contrast, transcription activities in plants exposed to red and far-red light were 30% to 85% less than in blue light/UV-A, depending on the gene and plant species. Total chloroplast, psbA, and 16S rrn transcription were 60% to 80% less in the Arabidopsis phyA mutant exposed to blue light/UV-A relative to wild type, thus definitively linking phyA signaling to these photoresponses. To our knowledge, the major role of phyA in mediating the blue light/UV-A photoresponses is a new function for phyA in chloroplast biogenesis at this stage of leaf development. Although rbcL expression in plants exposed to UV-A was 50% less in the phyA mutant relative to wild type, blue light-induced rbcL expression was not significantly affected in the phyA, phyB, and cry1 mutants. However, rbcL expression in blue light was 60% less in the phytochrome chromophore mutant, hy2, relative to wild type, indicating that another phytochrome species (phyC, D, or E) was involved in blue light-induced rbcL transcription. Therefore, at least two different phytochromes, as well as phytochrome-independent photosensory pathways, mediated blue light/UV-A-induced transcription of chloroplast genes in mature leaves.

The role of sigma factors in plastid transcription

Expression of plastid genes is controlled at both transcriptional and post-transcriptional levels in response to developmental and environmental signals. In many cases this regulation is mediated by nuclear-encoded proteins acting in concert with the endogenous plastid gene expression machinery. Transcription in plastids is accomplished by two distinct RNA polymerase enzymes, one of which resembles eubacterial RNA polymerases in both subunit structure and promoter recognition properties. The holoenzyme contains a catalytic core composed of plastid-encoded subunits, assembled with a nuclear-encoded promoter-specificity factor, sigma. Based on examples of transcriptional regulation in bacteria, it is proposed that differential activation of sigma factors may provide the nucleus with a mechanism to control expression of groups of plastid genes. Hence, much effort has focused on identifying and characterizing sigma-like factors in plants. While fractionation studies had identified several candidate sigma factors in purified RNA polymerase preparations, it was only 4 years ago that the first sigma factor genes were cloned from two photosynthetic eukaryotes, both of which were red algae. More recently this achievement has extended to the identification of families of sigma-like factor genes from several species of vascular plants. Now, efforts in the field are directed at understanding the roles in plastid transcription of each member of the rapidly expanding plant sigma factor gene family. Recent results suggest that accumulation of individual sigma-like factors is controlled by light, by plastid type and/or by a particular stage of chloroplast development. These data mesh nicely with accumulating evidence that the core sigma-binding regions of plastid promoters mediate regulated transcription in response to light-regime and plastid type or developmental state. In this review I will outline progress made to date in identifying and characterizing the sigma-like factors of plants, and in dissecting their potential roles in chloroplast gene expression.

Developmental stage-specific multi-subunit plastid RNA polymerases (PEP) in wheat

Most photosystem I and II plastid genes are transcribed by a plastid encoded Escherichia coli-like RNA polymerase (PEP). In this study, we show that both promoter selectivity and light-dependency of PEP change dramatically during development in wheat leaves. In the leaf tip, psbA and psbD promoter activities are light induced, whilst psbC, psbE and 16S rRNA promoters do not function efficiently irrespective of light conditions. In contrast to the leaf tip, in the basal portion all PEP promoters studied function in the dark as well as the light, except for psbD. Using in vitro transcription, we found that PEP in the illuminated leaf tip can initiate transcription from the -35 destructed psbA promoter, but the -35 element is essential for transcription in the basal portion. There is an extended -10 element in the psbA promoter, recognized by the PEP in the illuminated leaf tip or purified sigma 70-type Escherichia coli RNA polymerase but not by the PEP in the leaf base. These results suggest that during wheat leaf development, PEP in the leaf base that is functional for most PEP promoters even in the dark is replaced by the light-dependent PEP selectively transcribing the psbA and psbD promoters.

Detailed architecture of the barley chloroplast psbD-psbC blue light-responsive promoter

The photosystem II reaction center chlorophyll protein D2, is encoded by the chloroplast gene psbD. PsbD is transcribed from at least three different promoters, one which is activated by high fluence blue light. Sequences within 130 base pairs (bp) of the psbD blue light-responsive promoter (BLRP) are highly conserved in higher plants. In this study, the structure of the psbD BLRP was analyzed in detail using deletion and site-directed mutagenesis and in vitro transcription. Deletion analysis showed that a 53-bp DNA region of the psbD BLRP, from -57 to -5, was sufficient for transcription in vitro. Mutation of a putative prokaryotic -10 element (TATTCT) located from -7 to -12 inhibited transcription from the psbD BLRP. In contrast, mutation of a putative prokaryotic -35 element, had no influence on transcription. Mutation of a TATATA sequence located between the barley psbA -10 and -35 elements significantly reduced transcription from this promoter. However, site-directed mutation of sequences located between -35 and -10 had no effect on transcription from the psbD BLRP. Transcription from the psbD BLRP was previously shown to require a 22-bp sequence, termed the AAG-box, located between -36 and -57. The AAG-box specifically binds the protein complex AGF. Site-directed mutagenesis identified two different sequence motifs in the AAG-box that are important for transcription in vitro. Based on these results, we propose that positive factors bind to the AAG-box and interact with the chloroplast-encoded RNA polymerase to promote transcription from the psbD BLRP. Transcription from the psbD BLRP is thus similar to type II bacterial promoters that use activating proteins to stimulate transcription. Transcription of the psbD BLRP was approximately 6. 5-fold greater in plastid extracts from illuminated versus dark-grown plants. This suggests that light-induced activation of this promoter in vivo involves factors interacting with the 53-bp psbD BLRP in vitro.

DET1 represses a chloroplast blue light-responsive promoter in a developmental and tissue-specific manner in Arabidopsis thaliana

The chloroplast psbD-psbC loci, which encode the D2 and CP43 subunits of the photosystem II reaction center, respectively, are regulated by a blue light-responsive promoter (BLRP). It has recently been shown in barley seedlings that activation of psbD-psbC transcription by blue light involves inhibition of a protein kinase that represses the BLRP in the dark. To elucidate further the photosensory pathways regulating the psbD BLRP, the effects of three nuclear mutations on the expression of the BLRP in chloroplasts of Arabidopsis thaliana were examined. The mutants used included the det1-1 and det1-6 alleles for the nuclear protein DET1, involved in repressing photomorphogenesis, and the cry1 gene for the blue light photoreceptor, cryptochrome (CRY1), involved in hypocotyl elongation. The BLRP was not significantly expressed in cotyledons of light-grown wild-type seedlings, unlike the light-responsive expression of the chloroplast, psbA and rbcL, and nuclear, Lhcb and Chs, genes. Analysis of the mutants revealed that DET1 represses transcription from the BLRP in a developmental and tissue-specific manner, which is unique from the effects that DET1 has on other light-regulated promoters. In addition, the cry1 mutation did not reduce the expression of the BLRP in response to blue light. This suggests that the BLRP is regulated by a different photosensory system relative to CRY1. A model is proposed involving blue light, DET1 and phytochrome in regulating transcription from the psbD BLRP.