Structure and expression of sunflower ubiquitin genes

Two ubiquitin genes, designated UbB1 and UbB2, were isolated from a sunflower genomic library. They encode polyubiquitin transcripts corresponding to six repeats of the monomer. Northern blot analysis identified several different transcript size classes: both UbB1 and UbB2 transcripts are found in the most abundant 1.6 kb class. In contrast to the previously isolated UbF transcript which is present at high levels in flowers, UbB1 and UbB2 are expressed constitutively at low levels in different tissues. The levels of the two transcripts increase after heat stress. The two genes exhibit strong homology suggesting that they may result from duplication and conversion. Surprisingly, UbB1 gene shows structural similarities with the chicken ubiquitin heat shock gene, in particular the presence of an intron located just in front of the first ATG.