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Networking - a turn-on for some genes, a turn-off for others
Although we sometimes speak about signalling pathways, it is usually more appropriate to speak of signalling networks. This is well illustrated by the work of Ogawa et al. (Tokyo, Iwate and Saitama, Japan, pp. 239–244), which focuses on two genes in Arabidopsis thaliana. The first is APETALA2 (AP2), which is an A gene in the well-established ABC model for floral homeotic genes. The AP2 protein also contains two ethylene-responsive element binding protein domains, suggesting that in addition to acting as a transcription factor in flower formation it also regulates expression of at least some ethylene-responsive factor (ERF) genes. The other gene is an ERF gene, AtEBP, encoding a transcriptional activator, ethylene-responsive element binding protein. The gene is regulated by ethylene via the EIN2 protein but it may also be regulated by the AP2 transcription factor. The authors have therefore carried out a comprehensive series of experiments to test the interactions of the two genes. In the ap2-5 mutation, the effectiveness of the AP2 transcription factor is much reduced; this is clearly seen in flower development. In leaves, stems and flowers, the ap2-5 mutants also show increased levels of AtEBP mRNA and of AtERF1 (another ERF gene) mRNA. Thus, wild-type AP2 protein acts as a negative transcription factor for at least two ERF genes. Even more intriguingly, transcription of AP2 is increased in ap2-5 mutants: wild-type AP2 down-regulating its own gene. AtEBP also affects expression of AP2: in transgenic lines over-expressing the AtEBP gene, the amount of AP2 mRNA was increased (as was that of AtERF1). Thus, AtEBP is a positive regulator of AP2 gene expression. Based on these and other results in the paper, it is clear that the genes interact with each other in some ethylene responses, in flowering and in other aspects of plant development - a clear example of a signalling network.
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