Published online 22 May 2006
Article |
Multiple physical forms of excised group II intron RNAs in wheat mitochondria
Biology Department, University of Ottawa Ottawa, Canada K1N 6N5
*To whom correspondence should be addressed. Tel: +1 613 562 5800 ext. 6356; Fax: +1 613 562 5486; Email: lbonen{at}science.uottawa.ca
Received March 11, 2006. Revised April 3, 2006. Accepted April 13, 2006.
Plant mitochondrial group II introns do not all possess hallmark ribozymic features such as the bulged adenosine involved in lariat formation. To gain insight into their splicing pathways, we have examined the physical form of excised introns in germinating wheat embryos. Using RTPCR and cRTPCR, we observed conventional lariats consistent with a two-step transesterification pathway for introns such as nad2 intron 4, but this was not the case for the cox2 intron or nad1 intron 2. For cox2, we detected full-length linear introns, which possess non-encoded 3'terminaladenosines, as well as heterogeneous circular introns, which lack 3' nucleotide stretches. These observations are consistent with hydrolytic splicing followed by polyadenylation as well as an in vivo circularization pathway, respectively. The presence of both linear and circular species in vivo is supported by RNase H analysis. Furthermore, the nad1 intron 2, which lacks a bulged nucleotide at the branchpoint position, comprised a mixed population of precisely full-length molecules and circular ones which also include a short, discrete block of non-encoded nucleotides. The presence of these various linear and circular forms of excised intron molecules in plant mitochondria points to multiple novel group II splicing mechanisms in vivo.
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