The exon structure shown in the figure is deduced from comparison of genomic sequence to a 2.5 kb long cDNA clone, which has been isolated from a cDNA library kindly provided by Zs. Schwarz-Sommer and A. Gierl (MPI, Koln-Vogelsang). This cDNA clone extends into exon 3. Exons 1 and 2 have been identified and positioned by S1-mapping, primer extension and RNA sequencing experiments. An open reading frame encoding for a polypeptide of 802 amino acids starts in exon 2 and stops in exon 16. The introns begin with GT and end with AG. 31 bp in front of the polyA tail a polyadenylation signal AATAAA is found. Upstream (-29) of the transcription start a sequence TATTTATT has homology to the TATA-box.
From the evolutionary point of view, it is interesting to note that we find 17 bp exchanges at 3rd codon positions between the cDNA clone pWW 11071 (line C) and our genomic clone (material from McClintock, Ds at standard position). None of these base substitutions causes an amino acid exchange. Calculations using base substitution rates derived from animal genes indicate that these two alleles would be more than 3 million years apart. Since present day maize is much younger, our observation could be explained either by an exceptionally high mutation rate in maize or by the assumption that the allelic diversity of maize ancestors has been introduced into Zea mays, as discussed by Galinat (1977, Corn and Corn Improvement). At the 3'-end we find two small duplications, which could be the consequence of the insertion and subsequent excision of transposable elements, as has been suggested by Schwarz-Sommer et al. (EMBO J., in press).
W. Werr, W-B. Frommer, C. Maas, and P. Starlinger
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