We have also studied the expression of this fusion gene in maize tissues. An ubiquitin fusion gene clone was identified by screening a lambda maize W22 genomic library with a short oligonucleotide probe derived from the sequence for a yeast ubiquitin fusion gene. A 5 kb Bam HI fragment containing the ubiquitin fusion gene sequence was subcloned into the pUC119 vector. The deduced amino acid sequence of the ubiquitin portion is identical to the sequence of the maize ubiquitins we have previously studied. The gene is transcribed as a 900-nucleotide transcript.
Northern blots indicate that the gene is expressed at various levels in different maize tissues.
The expression of the fusion gene appears to be regulated during endosperm development. W22 DNA demonstrate that fusion gene sequences are present in more than one copy in the maize genome. The promotor region of the fusion gene has been characterized by sequence analysis and S1 mapping of the transcription initiation site. An 800 nucleotide fragment of the promotor sequence has been fused to a GUS reporter gene to study the regulation of the fusion gene's expression. The GUS gene under the control of the fusion gene promotor has expressed.