S2F, a leaf-specific trans-acting factor, binds to a novel cis-acting element and differentially activates the RPL21 gene

The rice TCD11 encoding plastid ribosomal protein S6 is essential for chloroplast development at low temperature

Plastid ribosome proteins (PRPs) are important components for chloroplast biogenesis and early chloroplast development. Although it has been known that chloroplast ribosomes are similar to bacterial ones, the precise molecular function of ribosomal proteins remains to be elucidated in rice. Here, we identified a novel rice mutant, designated tcd11 (thermo-sensitive chlorophyll-deficient mutant 11), characterized by the albino phenotype until it died at 20°C, while displaying normal phenotype at 32°C. The alteration of leaf color in tcd11 mutants was aligned with chlorophyll (Chl) content and chloroplast development. The map-based cloning and molecular complementation showed that TCD11 encodes the ribosomal small subunit protein S6 in chloroplasts (RPS6). TCD11 was abundantly expressed in leaves, suggesting its different expressions in tissues. In addition, the disruption of TCD11 greatly reduced the transcript levels of certain chloroplasts-associated genes and prevented the assembly of ribosome in chloroplasts at low temperature (20°C), whereas they recovered to nearly normal levels at high temperature (32°C). Thus, our data indicate that TCD11 plays an important role in chloroplast development at low temperature. Upon our knowledge, the observations from this study provide a first glimpse into the importance of RPS6 function in rice chloroplast development.

Cloning and transcriptional activity analysis of the porcine cofilin 2 gene promoter

Cofilins (CFL), including CFL1 and CFL2, are members of the family of actin-binding proteins in eukaryote. CFL2 is predominantly expressed in mammalian skeletal muscle and heart and is important to muscle fiber formation and muscular regeneration. To study transcriptional regulation of porcine CFL2, a 2.5 kb upstream sequence starting from the major CFL2 transcriptional start site was cloned by genome walking. Twelve DNA fragments of the 5' flank region of the porcine CFL2 gene were further isolated from porcine genomic DNA via PCR and inserted into the luciferase reporter vector pGL4.10 to make 12 CFL2 reporter constructs. All reporter vectors were transfected into C2C12, NIH3T3, or Hela cells and their relative luciferase activity measured after 48 h, respectively. Bioinformatics analysis suggested that there were two TATA-boxes at the -508 bp and -453 bp, as well as a GC-box and a CAAT-box in this sequence. Additional transcription factor binding sites including SP1, AP1, AP2, and GATA-1 sites were also predicted. The transcriptional activity of pGL4.10-1554 (1502 bp to +51 bp) was the strongest, and the promoter's active region was mapped to a region from -1502 bp to -1317 bp. Our data provide a foundation for future studies into transcriptional regulation of CFL2.

The chloroplast ribosomal protein L21 gene is essential for plastid development and embryogenesis in Arabidopsis

Embryogenesis in higher plants is controlled by a complex gene network. Identification and characterization of genes essential for embryogenesis will provide insights into the early events in embryo development. In this study, a novel mutant with aborted seed development (asd) was identified in Arabidopsis. The asd mutant produced about 25% of albino seeds at the early stage of silique development. The segregation of normal and albino seeds was inherited as a single recessive embryo-lethal trait. The gene disrupted in the asd mutant was isolated through map-based cloning. The mutated gene contains a single base change (A to C) in the coding region of RPL21C (At1g35680) that is predicted to encode the chloroplast 50S ribosomal protein L21. Allele test with other two T-DNA insertion lines in RPL21C and a complementation test demonstrated that the mutation in RPL21C was responsible for the asd phenotype. RPL21C exhibits higher expression in leaves and flowers compared with expression levels in roots and developing seeds. The RPL21C-GFP fusion protein was localized in chloroplasts. Cytological observations showed that the asd embryo development was arrested at the globular stage. There were no plastids with normal thylakoids and as a result no normal chloroplasts formed in mutant cells, indicating an indispensable role of the ASD gene in chloroplasts biogenesis. Our studies suggest that the chloroplast ribosomal protein L21 gene is required for chloroplast development and embryogenesis in Arabidopsis.

Analysis of the alternative oxidase promoters from soybean

Alternative oxidase (Aox) is a nuclear-encoded mitochondrial protein. In soybean (Glycine max), the three members of the gene family have been shown to be differentially expressed during normal plant development and in response to stresses. To examine the function of the Aox promoters, genomic fragments were obtained for all three soybean genes: Aox1, Aox2a, and Aox2b. The regions of these fragments immediately upstream of the coding regions were used to drive beta-glucuronidase (GUS) expression during transient transformation of soybean suspension culture cells and stable transformation of Arabidopsis. The expression patterns of the GUS reporter genes in soybean cells were in agreement with the presence or absence of the various endogenous Aox proteins, determined by immunoblotting. Deletion of different portions of the upstream regions identified sequences responsible for both positive and negative regulation of Aox gene expression in soybean cells. Reporter gene analysis in Arabidopsis plants showed differential tissue expression patterns driven by the three upstream regions, similar to those reported for the endogenous proteins in soybean. The expression profiles of all five members of the Arabidopsis Aox gene family were examined also, to compare with GUS expression driven by the soybean upstream fragments. Even though the promoter activity of the upstream fragments from soybean Aox2a and Aox2b displayed the same tissue specificity in Arabidopsis as they do in soybean, the most prominently expressed endogenous genes in all tissues of Arabidopsis were of the Aox1 type. Thus although regulation of Aox expression generally appears to involve the same signals in different species, different orthologs of Aox may respond variously to these signals. A comparison of upstream sequences between soybean Aox genes and similarly expressed Arabidopsis Aox genes identified common motifs.

Architecture and transcriptional activity of the initiator element of the TATA-less RPL21 gene

The nuclear RPL21 gene coding for the plastid ribosomal protein L21 is a TATA-less gene that is overexpressed in a leaf-dependent manner by the specific usage of a strong initiator called P1. We have previously shown that the RPL21 core promoter spanning from -23 to +104 relative to P1 start site activates transcription in the same manner as does the full promoter. Here, we present results of experiments aimed at deciphering the RPL21 core promoter architecture. Results of transient expression using various 5' deletions of the core promoter fused to a chloramphenicol acetyl transferase (CAT) reporter gene show that 34 bp encompassing the P1 initiation site (from -23 to +11) are required for full transcription activation. Gel-shift analysis shows that five DNA/protein complexes (C1-C5) are formed on this 34-bp fragment with protein extracts from green tissues. C1 is the major complex present during seed germination. The other complexes are present in young leaf tissues suggesting a role in transcription activation. Linker scanning mutagenesis experiments show that the five complexes form two independent groups: I (C1-C3) and II (C4 and C5), with a common binding site located on P1. Using transgenic plants, we show that three nucleotides encompassing the P1 start site and three trinucleotides necessary for group I binding are determinant for RPL21 activation. These results identify an unusually compact core structure, which is centred on P1 initiation site and is responsible for transcription activation. A model of the architecture of this region is presented.

A nuclear transcription factor related to plastid ribosome biogenesis is synthesised early during germination and priming

Germination is a short developmental process during which many new proteins are synthesised. We have chosen the previously characterised RPL21 gene encoding plastid-localised ribosomal proteins RPL21, to analyse activation of gene expression during germination. Transcription activation occurs at the P1 promoter during the first hours following imbibition and coincides with the appearance of a trans-acting factor that we named AUBE1. AUBE1 binds specifically to a short DNA fragment that encompasses the P1 promoter of the RPL21 gene. The protein has a size of 28-30 kDa and is transiently expressed during the early phase of germination. Using the properties of primed seeds we show that AUBE1 is maintained after desiccation of primed seeds. We conclude that AUBE1 can be used as a marker in spinach seed priming.

The Arabidopsis chloroplast ribosomal protein L21 is encoded by a nuclear gene of mitochondrial origin

Many chloroplast genes of cyanobacterial origin have been transferred to the nucleus during evolution and their products are re-addressed to chloroplasts. The RPL21 gene encoding the plastid r-protein L21 has been lost in higher plant chloroplast genomes after the divergence from bryophytes. Based on phylogenetic analysis and intron conservation, we now provide evidence that in Arabidopsis a nuclear RPL21c gene of mitochondrial origin has replaced the chloroplast gene. The control of expression of this gene has been adapted to the needs of chloroplast development by the acquisition of plastid-specific regulatory promoter cis-elements.