Auxins are plant hormones that play a central role in controlling plant growth and development across different environmental conditions. Even at low concentrations, auxins can regulate gene expression through specific transcription factors and proteins that are modulated to environmental responses in the signalling cascade. Auxins are synthesized in tissues with high cell division activity and distributed by specific transmembrane proteins that regulate efflux and influx. This review presents recent advances in understanding the biosynthetic pathways, both dependent and independent of tryptophan, highlighting the intermediate indole compounds (indole-3-acetamide, indole-3-acetaldoxime, indole-3-pyruvic acid and tryptamine) and the key enzymes for auxin biosynthesis, such as YUCs and TAAs. In relation to the signalling cascade, it has been shown that auxins influence gene expression regulation by the connection between synthesis and distribution. Moreover, the molecular action of the auxin response factors and auxin/indole-3-acetic acid transcription factors with the F-box TIR1/AFB auxin receptors regulates gene expression. In addition, the importance of microRNAs in the auxin signalling pathway and their influence on plant plasticity to environmental fluctuations is also demonstrated. Finally, this review describes the chemical and biological processes involving auxins in plants.
Developmental regulation of y-globin gene expression probably occurs through developmental-stagespecific trans-acting factors able to promote the interaction of enhancer elements located in the far upstream locus control region with regulatory elements in the y gene promoters and 3' At enhancer located in close proximity to the genes. We have detected a nuclear protein in K562 and baboon fetal bone marrow nuclear extracts capable of binding to A+T-rich sequences in the locus control region, y gene promoter, and 3' A7 enhancer. SDS/polyacrylamide gel analysis of the purified K562 binding activity revealed a single protein of 87 kDa. A K562 cDNA clone was isolated encoding a fi-galactosidase fusion protein with a DNA binding specificity identical to that of the K562/fetal bone marrow nuclear protein. The cDNA clone encodes a homeodomain homologous to the Drosophila antennapedia protein.developmental-stage-specific factor presumably mediating this interaction in chickens has been demonstrated (15).The binding of erythroid and tissue-ubiquitous factors to sequences present in the y-globin promoter (16-21), the human P-globin promoter (22), and the human f3-globin 3' enhancer (23) have been described. No developmentalspecific factor for the human globin gene has been reported (13). We describe a factor in K562 and fetal baboon bone marrow (BM) nuclear extracts binding specifically to sequences in the 3' Aly enhancer, y-globin promoter, and LCR.A clone obtained from a Agtll K562 cDNA library expresses a f-galactosidase (J-gal) fusion protein that is also able to bind specifically to these sequences. The cDNA insert of this clone encodes a homeodomain homologous to the Drosophila antennapedia protein.*The 8-globin complex, in humans and higher-order primates, is a cluster offive genes in the order 5' _ 3' whose expression normally occurs only in erythroid cells. Expression ofthe y-globin gene in humans and higher order primates occurs primarily in fetal life and is replaced by expression of the B-globin gene postnatally (1). Activation of dormant globin genes in cell hybrids (2) and heterokaryons (3) indicates that expression of globin genes can be positively regulated by diffusible trans-acting factors.Sequences that presumably interact with trans-acting factors to determine the developmental switch in humans have been defined by data from transgenic mouse experiments. The locus control region (LCR), originally identified by the presence oferythroid-specific DNase I hypersensitive sites in a region 20 kilobases 5' to the E-globin gene (4), confers high-level expression to both globin and heterologous promoters in transgenic mice (5). Correct developmental switching from y-to P3-globin gene expression in transgenic mice occurs when both the y-and ,3-globin genes are linked in cis to the LCR (6, 7). Promoter sequences of both -y-globin genes (8, 9) and an enhancer 3' to the 3-globin gene (10, 11) are required for stage-specific gene expression in the transgenic mouse. A sequence 3' to the Ay gene has the ability...
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