Characterization of the structure and transcription of an ubiquitin fusion gene from maize

We have identified a maize ubiquitin (Ubi) fusion gene (UBF9) by screening a maize W22 genomic phage λ library with a short (16-nucleotide) oligodeoxyribonucleotide probe derived from the sequence for the extension sequence of a yeast UBI3 fusion gene. UBF9 consists of an UB monomer sequence (228 bp long) joined to an extension sequence (237 bp long). The extension sequence encodes a protein of 79 amino acids which shares extensive identity with similar extension aa sequences found in yeast, humans, barley and Arabidopsis thaliana. UBF9 encodes a small-size class of Ubi mRNAs in the maize tissues investigated. The UBF9 transcript is present in high levels in maize endosperm tissues 22 days after pollination. Genomic Southern blots of maize inbred W22 DNA indicate that the fusion gene sequences are present in multiple copies in the maize genome. Primer extension experiments indicate that the transcription start point is located at 80 bp upstream from the translation start codon of UBF9, Two 37-bp tandem repeated A + T-rich sequences are found in the 5'-flanking region of UBF9. The A + T-rich sequences share the motif, AATATTTTATT, which is present in a diverse set of plant genes.