The homeodomain is a DNA-binding domain present in a large family of eukaryotic regulatory proteins. Homeodomain proteins have been shown to play key roles in controlling developmental programs in various organisms. Here we report the isolation and characterisation of a homeobox gene from Arabidopsis thaliana designated ATK1. The gene was isolated using as a probe the homeobox domain of the KN1 gene from maize. The homeodomain of ATK1 is highly homologous to the homeodomain of the KN1 gene of maize (81%) but shows only poor homology outside the homeodomain. Therefore ATK1 is probably not the Arabidopsis homologue of the KN1 gene from maize. It contains the four invariant amino acid residues present in the recognition helix 3 of all other homeodomain proteins. Outside the homeodomain a region rich in aspartate and glutamate residues is found suggesting that ATK1 is a transcriptional activator. The gene contains four introns which is similar in the KN1 gene of maize and the Osh1 gene of rice. Primer extension reveals the presence of two transcription initiation sites. The leader sequence of the genuine transcript is 342 nucleotides long and contains two upstream open reading frames. ATK1 is strongly expressed in the shoot apex of seedlings, while in mature plants the gene is primarily expressed in flowers and inflorescence stems. Such an expression pattern is reminiscent of that of the KN1 gene of maize and therefore ATK1 could similarly be involved in determining cell fate.
Plastocyanin is part of the photosynthetic electron transport chain in the chloroplast and is encoded in the nucleus. Expression of the Arabidopsis thaliana plastocyanin gene is organ specific: high mRNA levels are observed in young green parts of the plant. Furthermore, expression is dependent on the presence of light and functional chloroplasts. When grown in the presence of norflurazon under white light conditions, resulting in the photo-oxidative destruction of the chloroplast, plastocyanin mRNA levels are strongly reduced. A -1579 to -9 promoter fragment confers light-regulated and chloroplast-dependent expression to the beta-glucuronidase reporter gene in transgenic tobacco plants. This suggests that regulation takes place at the level of transcription. A plastocyanin promoter deletion series ranging from -1579 to -121 which was also tested in tobacco, revealed the presence of a strong positive regulating element (PRE) in the -1579 to -705 region. Deletion of this part of the promoter resulted in a approximately 100-fold reduction of GUS expression as measured in mature leaves. Surprisingly, this enhancer-like element was capable of stimulating transcription from a position downstream of its reporter. Moreover, it could also activate a truncated CaMV 35S promoter. Deletion of this element coincides with the loss of chloroplast-dependency of reporter gene expression, as judged by norflurazon treatment of transgenic seedlings. So, the activity of the PRE itself might depend on the presence of functional chloroplasts.
The plastocyanin (PC) gene of Arabidopsis fhaliana is activated independently of light during early seedling development. In etiolated seedlings, PC mRNA levels increase transiently and a maximum dark leve1 is reached after 2 d of growth in darkness. In etiolated transgenic seedlings carrying a chimeric PC-promoter: luciferase fusion gene, luciferase activity is similarly increased after 2 d of growth. The transient increase in PC mRNA and luciferase activity levels can be repressed by sucrose. Nonmetabolizable sugars and polyethylene glycol do not have a major effect on PC gene expression. Also, light-grown seedlings show a similar transient and sucrose-sensitive increase in PC mRNA levels and luciferase activity, as in dark-grown seedlings, but here expression levels are 15-fold higher. These findings suggest the presence of a sucrose-sensitive, developmentally controlled expression mechanism that operates independently of light.
Seven different MYB-related genes have been isolated from a genomic Arabidopsis library with probes based on MYB DNA-binding motifs. The predicted amino acid sequence of these genes showed high similarity in the MYB domain but outside this region virtually no similarities were found. The set of MYB-related genes was used to identify differentially expressed genes following the transfer of etiolated seedlings to light. This differential screen resulted in the selection of the ATM4 gene which is induced by light within one hour of exposure of etiolated or dark-adapted seedlings.
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