Michael MulliganProfessor, Developmental & Cell Biology |
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Research Interests |
RNA editing in plants and RNA processing in gene expression | |
| URL | devcell.bio.uci.edu/html/FACULTY%20CVs/mulligan.html | |
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Research Abstract |
My lab is interested in expression organellar genomes in plants. The major focus over the last decade has been on RNA editing in plant mitochondria. The two major aspects of RNA editing that we have addressed have been (1) the consequences of incomplete editing to gene expression and (2) the mechanism of RNA editing. RNA editing causes nucleotide changes in the sequence of RNAs, and consequently changes the coding information of the transcript. The consequences of incomplete editing had been a controversial issue. First, transcripts for many genes are fully edited, and these transcripts encode the evolutionarily conserved amino acid sequence and direct the synthesis of functional polypeptides. Second, nascent transcription products such as pre-mRNAs show a high degree of heterogeneity with respect to editing status, and examples of cDNAs at every stage of editing may be detected; however, the mature spliced transcripts for these genes are completely edited such that gene expression is not compromised. Third, transcripts for some genes, such as rps12 and rps13, are only 50 to 80 % edited, respectively (Phreaner et al., 1996; Williams et al., 1998a). The incompletely edited transcripts tend to be highly transcribed and incomplete editing may result from a short half life of these transcripts coupled with a relatively slow rate of relative editing. A major issue in the area of gene expression was whether incompletely edited RNAs were utilized as templates for translation. In order to directly establish the nature of the translation products of incompletely edited transcripts, we initiated a project to produce antibodies that discriminated between edited and unedited translation products. Analysis with the edited rps12 antibodies demonstrated that the edited translation products were in fact translated and that the edited polypeptides accumulated mitochondrial ribosomes. The more interesting result was obtained with the unedited antibody which demonstrated for that unedited rps12 transcripts were translated, and that the unedited RPS12 polypeptides accumulate in the mitochondrial matrix, but do not assemble into mitochondrial ribosomes in maize mitochondria (Phreaner et al., 1996). This was the first demonstration that unedited transcripts were in fact translated. One of the outstanding questions is how editing sites are specified. Editing sites are usually nearly completely converted; however nearby cytidine residues are never converted. Thus, a mechanism exists to identify which Cs should be converted, and an understanding of the cis-acting sequences in the RNA may lead to the identification of a specificity factor. Genetic transformation of plant mitochondria has not been demonstrated, and therefore analysis of the editing of recombinant transcripts is not possible; however plant mitochondrial genomes undergo spontaneous recombination and "fragment" the genome. rps12 has participated in homologous recombination in maize mitochondria and a second copy of rps12 has been created. Fortuitously, the recombination occurred within two 7 to 9 nucleotide sequences which actually included an editing site in each recombination site. Thus, this spontaneous recombination created chimeric editing substrates that could be analyzed to infer the cis-acting sequences which confer editing site recognition (Williams et al., 1998b). The results from this work suggest that 5’ flanking RNA sequences are critical for editing site recognition, but 3’ flanking sequences are not important. We are using information from these cis-acting sequence to attempt to identify antisense gRNAs that could be involved in editing site recognition. |
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| Publications | Yang, A. J. and R. M. Mulligan. (1991) RNA Editing Intermediates of cox2 Transcripts in Maize Mitochondria. Molecular and Cellular Biology: 11: 4278-4281. | |
| Phreaner, C.G., Williams, M.W., and R. M. Mulligan. (1996) Incomplete Editing of rps12 Transcripts Results in the Synthesis of Polymorphic Polypeptides in Plant Mitochondria. The Plant Cell 8:107-117 | ||
| Lu, B., Wilson, R.K., Phreaner, C.G., Mulligan, R.M., and M.R. Hanson. (1996) Protein Polymorphism Generated by Differential RNA Editing of a Plant Mitochondrial rps12 Gene. Molecular and Cellular Biology 16: 1543 - 1549. | ||
| Williams, M.A., Tallakson, W.A. Phreaner, C.G., and R.M. Mulligan (1998a) Editing and Translation of Ribosomal Protein S13 Transcripts: Unedited Translation Products Are Not Detectable in Maize Mitochondria. Current Genetics 34:221-226 | ||
| Williams, M.A., Kutcher, B.M., and R.M. Mulligan (1998b) Editing Site Recognition in Plant Mitochondria: the Importance of 5' Flanking Sequences. Plant Molecular Biology 36:229-237 | ||
| Graduate Programs |
Cell Biology |
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| Link to this profile | http://www.faculty.uci.edu/profile.cfm?faculty_id=3342 | |
| Last updated | 08/03/2005 | |