Ubiquitin is a 76-amino-acid protein found in all eukaryotic cells. Its amino acid sequence is highly conserved and, in most cases, is derived from a multigene family. The genes coding for ubiquitin appear to consist of a variable number of repeated ubiquitin coding elements which are not separated by introns or other spacer sequences. Transcripts from these genes consist of large mRNAs containing multiple copies of information coding for ubiquitin. These 'polycistronic-like' mRNAs are translated into 'polyubiquitin' proteinaceous products which must be split precisely to produce the final functional ubiquitin.
Although the general structure of the ubiquitin genes is conserved, there are species differences in the number of genes per genome, in the number of repeats and in the size of the mRNA transcripts. As a prelude to characterizing the ubiquitin genes and the regulation of their expression in maize, we determined (by use of an avian ubiquitin cDNA probe) the size and level of ubiquitin transcripts in the radicles and plumules of control and heat-shocked (25 - >42.5 C) 5-day-old Oh43 maize seedlings. Results from these Northern analyses disclosed striking differences in the size of the ubiquitin mRNAs constitutively synthesized in each organ. Plumules produced ubiquitin mRNAs consisting of 1200-2000 nucleotides while primary radicles from the same seedlings produced ubiquitin mRNAs which range from 700-1500 nucleotides in length. Although the Northern analyses did not detect any changes in the organ-specific size of the ubiquitin mRNAs from heat-shocked seedlings, quantitation from dot-blots reveals that heat shock causes a marked increase in radicle (4-fold) and plumule (2-fold) ubiquitin mRNA levels. These results suggest (1) that maize ubiquitin is, as it is in animal cells, a heat shock protein, and (2) that the ubiquitin synthesized by the radicles and plumules of control or heat-shocked 5-day-old maize seedlings are either the products of different genes being expressed in each organ or are products of the same gene which undergo organ-specific processing.
Ling Liu*, D.B. Walden and B.G. Atkinson*
*Department of Zoology
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