STRUCTURE AND PROMOTER ACTIVITY OF A STRESS AND DEVELOPMENTALLY-REGULATED POLYUBIQUITIN-ENCODING GENE OF NICOTIANA-TABACUM

A polyubiquitin-encoding gene was identified from a Nicotiana tabacum genomic library using a specific probe spanning the 3' untranslated re gion of the corresponding cDNA. The gene, Ubi.U4, is expressed in vari ous amounts in the whole plant, except in just-fully-expanded leaves. Genomic blots indicate that it originates from N. tomentosiformis. Seq uence analyses reveal that the gene consists of four ubiquitin monomer s extended by a fifth truncated subunit. It is disrupted by a single 4 57-bp intron in close proximity to the start codon of translation. Pri mer extension experiments localized the transcription start point (tsp ). Transient gene expression in N. tabacum protoplasts indicates that the deletion of the intron has no significant influence on gene expres sion. Mutagenesis on putative cis-regulatory elements indicates at lea st three important motifs in the proximal promoter: an 'ACGT' core ele ment, an A+T-rich sequence and a less clearly defined cis-element loca ted between bp -162 and -113.