INDEPENDENT MODULATION OF ARABIDOPSIS-THALIANA POLYUBIQUITIN MESSENGER-RNAS IN DIFFERENT ORGANS AND IN RESPONSE TO ENVIRONMENTAL-CHANGES

The highly conserved protein ubiquitin is encoded by five polyubiquiti n genes in Arabidopsis thaliana ecotype Columbia that have been divide d into two subtypes, the UBQ3/UBQ4 subtype and the UBQ10/UBQ11/UBQ14 s ubtype. Northern analysis using gene-specific oligonucleotides as hybr idization probes and enzyme activity measurements from transgenic plan ts expressing beta-glucuronidase (GUS) under the control of individual polyubiquitin 5' flanking regions were used to determine the developm ental and environmental regulation of polyubiquitin transcription and mRNA accumulation. Polyubiquitin mRNA levels within and between subtyp es were independently modulated. UBQ3 mRNA levels were threefold highe r than UBQ4 mRNA levels in vegetative organs, but only two-thirds of t he UBQ4 mRNA levels in flowers. UBQ3 mRNA was modulated by dark/light treatments, while mRNAs from UBQ4 and all members of the other subtype were unaffected. Similarly, within the UBQ10/ UBQ17/UBQ14 subtype, UB Q11/UBQ14 mRNAs were modulated differently in seedlings after a two-ho ur heat-shock treatment. Among all the polyubiquitin genes, UBQ10 mRNA level was the most constant in all organs and environmental condition s examined. Transgenic plants transformed with a UBQ10 5' flanking reg ion::GUS gene contained higher levels of GUS activity than transgenic plants expressing GUS under the control of UBQ3 5' flanking regions. I n conclusion, the relative abundance of different Arabidopsis polyubiq uitin mRNAs, even those produced from highly similar genes within a su btype, appears to he modulated independently in response to developmen tal and environmental cues.