Submitted to: Genetics
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/4/2002
Publication Date: 2/1/2003
Citation: Cui, X., Hsia, A., Liu, F., Ashlock, D.A., Wise, R.P., Schnable, P.S. 2003. Alternative transcription initiation sites and polyadenylation sites are recruited during Mu suppression at the rf2a locus of maize. Genetics. 163:685-698. Interpretive Summary: Maize hybrids have a significant yield advantage over open-pollinated varieties. One way of producing hybrid seed efficiently is through the use of cytoplasmic male-sterile (CMS) germplasm. Since CMS plants do not produce viable pollen, they are unable to cross pollinate with other plants. This is an advantage because it eliminates the need for the time and labor intensive task of hand emasculation of the inbred lines. Most CMS systems also have restorer genes that correct the defect of male sterility. By using a combination of characterized cytoplasmic male- sterile plants and plants which contain nuclear restorer genes, one can produce hybrid seed effectively. However, our understanding of the way restoration occurs has been hampered by the lack of information on the mechanism by which nuclear restorer genes function. In this manuscript, we report the suspension of transposable DNA element induced (Mu) that were used to clone the rf2 restorer gene from T-cytoplasm maize. Mu suppression has the potential to complicate efforts to clone genes via transposon tagging, a widely used technique for isolation of genes of agronomic importance. Because suppressed plants exhibit a wild-type phenotype even though they carry a Mu transposon in the target gene, suppression can mask the desired trait. Mu suppression can also lead to the loss of mutant traits during backcrossing programs that are becoming increasingly important with the adoption of new biotechnologies such as RNA profiling and proteomics. This is the first description of Mu suppression affecting a gene that corrects the male sterile trait in corn. The results described in this manuscript will facilitate research of other scientists working in plant development and molecular breeding.
Technical Abstract: Even in the absence of excisional loss of the associated Mu transposons, some Mu-induced mutant alleles of maize can lose their capacity to condition a mutant phenotype. Three out of five Mu-induced rf2 alleles are susceptible to Mu suppression. The suppressible rf2-m9437 allele has a novel Mu transposon insertion (Mu10) in its 5' untranslated region (UTR). The suppressible rf2-m9390 allele has a Mu1 insertion in its 5' UTR. During suppression, alternative transcription initiation sites flanking the Mu1 transposon yield functional transcripts. The suppressible rf2-m8110 allele has an rcy:Mu7 insertion in its 3' UTR. Suppression of this allele occurs via a previously unreported mechanism; sequences in the terminal inverted repeats of rcy:Mu7 function as alternative polyadenylation sites such that the suppressed rf2-m8110 allele yields functional rf2 transcripts. The nucleotide compositions of these alternative polyadenylation sites were compared with 94 other maize polyadenylation sites. No significant difference was found at the 95% confidence level (c2 =3.67). In contrast to earlier reports, this study revealed only a weak correlation between hyper-methylation of Mu transposons and Mu suppression.