The cDNA fragments of auxin-binding protein (ABP1) were isolated by degenerated-primer mediated reverse transcription-polymerase chain reaction (RT-PCR) from dicotyledonous plant species, i.e. cress, mung bean, pea, radish and soybean, and monocotyledonous species, i.e. oat and rice. Cloned abp 1 cDNA fragments were sequenced and compared with known abp 1 clones from A. thaliana, maize, strawberry and tobacco at the amino acid level. In all plant species studied, the amino acid sequences of newly isolated abp 1 clones were highly conserved at two regions (region A: Thr-Pro-Ile-His-Arg-His-Ser-Cys-Glu-Glu-Ile/Val-Phe-Ile/Thr/Val-Val-Leu/Pro/V al-Lys-Gly-Xaa-Gly-Thr-Leu/Val; region B: His-Glu-Asp-Leu-Gln-Phe/Val-Leu-Asp/Val-Ile/Val-Ile-Ser-Arg-Pro-Pro), which were previously reported to be important for auxin-binding. On the other hand, some putative residues of amino acids in the region between regions A and B could be found to be specific in dicot and monocot species, respectively. Southern-blot analysis indicated a small abp 1 gene family in all species studied. Northern-blot analysis indicated that the size of abp 1 mRNA transcripts in all species studied was conserved at approximately 850 bp. The phylogenetic tree of ABP 1 was analyzed by the UPGMA method.
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