Ling Liu

Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Ubiquitin is a highly conserved, 76 amino acid, heat-stable protein found in many eukaryotic cells, including Zea mays L. Northern and dot blot hybridization analyses, using the ubiquitin coding sequence as a probe, demonstrate that the ubiquitin genes in maize are constitutively expressed at a control temperature (25{dollar}\sp\circ{dollar}C), and that heat shock (42.5{dollar}\sp\circ{dollar}C) induces an increase in polyribosome-associated ubiquitin mRNAs in the radicles (4-fold) and plumules (2-fold) of 5-day-old maize (inbred: Oh43) seedlings. These data imply that maize ubiquitin is a heat shock protein.;To assess the implication that maize ubiquitin is a heat shock protein encoded from genes whose expression is induced by heat shock, sequences encoding five different maize ubiquitin genes were isolated, cloned, identified, and characterized. The results reveal that ubiquitin arises from a family of genes and that the mRNAs encoded from these maize genes are polycistronic, containing either tandem repeats of a ubiquitin coding region (polyubiquitin) or a single ubiquitin coding region fused to a sequence coding for an extension protein (ubiquitin fusion protein). Three of the mRNAs, containing sequences derived from clones MubC1, MubC5, and MubC9, encode 5 or 7 repeats of the ubiquitin sequence, and are approximately 2.0, 2.0 and 1.7 kb in size. The other two mRNAs, containing sequence derived from clones MubG7 and MubG10, are approximately 0.7 kb in size, and each encodes a single ubiquitin sequence and a single, but different extension protein sequence. Northern and dot blot hybridization analyses, using sequences specific for each of these ubiquitin-containing genes (rather than a sequence from the common ubiquitin coding region), reveal, in fact, that individual members of this gene family respond differently to heat shock. Indeed, on the bases of their response to heat shock, members of the maize ubiquitin gene family can be classified into three groups: (a) those whose transcripts accumulate on polyribosomes during heat shock (e.g. MubC1 and MubC5); (b) those whose transcripts decrease on polyribosomes during heat shock (e.g. MubG7 and MubG10); and (c) those whose association with polyribosomes does not change during heat shock (e.g. MubG9).



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