Both petunia an3 class2 alleles and the maize o2-hf allele are characterized by a high frequency of reversions. Reversion of the class 2 petunia alleles occurs by epigenetic interaction between three dTph1 elements, situated in two homologous chromosomes (van Houwelingen, 1999). The high frequency of reversion of the o2-hf allele is characterized by the epigenetic interaction as well, but between the receptor element rbg, present in this allele, and the regulatory element Bg-hf (Koterniak, Genetika, in press). The similarity between the maize and petunia alleles mentioned could indicate a similarity in the mechanisms of their reversions. Therefore, by analogy with the class 2 an3 petunia alleles, in the case of the o2-hf allele, at or near the o2 locus, two receptor rbg elements could be expected. The loss of one dTph1 element leads to suppression of the class 2 allele reversions (van Houwelingen, 1999). Existence of the analogous suppression for the o2-hf allele in case of the loss of one of the rbg elements could explain significant variability in revertant content and effectiveness of reverse selection for revertant frequency observed in the strains containing this allele (Koterniak, in press; Koterniak, MNL 75:51-53, 2001).
Reversion of the petunia class1 an3 alleles is characterized by the absence of epigenetic effects and by a low frequency (van Houwelingen, 1999) similar to the o2-lf allele. It is necessary to mention that excisions of dTph1 elements from the class 2 an3 alleles, in contrast to their excisions from the class 1 alleles, lead to a perfect restoration in a majority of cases of the wild-type gene sequence (van Houwelingen, 1999). We expect that the determination of the character of restoration of the wild-type sequence in the case of the rbg reversion from the o2-lf and o2-hf alleles could confirm similarity in reversion mechanisms of the maize and petunia alleles mentioned.
Similarity in dosage effects expression between the regulatory elements Bg and Ac. Earlier it was reported that two regulatory elements, Bg-lf and Bg-hf, significantly differ in their ability to cause rbg excision in different doses (Koterniak, 1999). For the Bg-hf (in contrast to the Bg-lf) a strong positive dependence was characteristic between its dose and the frequency of revertants.
However, further studies show that there exists a non-linear and quite complicated dependence (Koterniak, in press; unpublished data) between the frequency of revertants and the dose of the Bg regulatory elements. As in earlier experiments, the frequency of revertants observed at 1 dose of the regulatory element Bg-hf was much lower than the frequency observed at 2 and 3 doses (revertant frequency conditioned by one dose may be on the same level with the revertant content observed in the crosses involving the regulatory element Bg-lf and the o2-lf allele) (Koterniak, 1999; Koterniak, in press). Increase in the dose of the Bg-hf from 1 to 2 or 3 increases the frequency of revertant formation by 6-19 times (calculating per one dose of the regulatory element) (Koterniak, 1999; Koterniak, in press). However, the increase in Bg-hf doses from 2 to 3 does not lead to significant enhancement of revertant content; moreover, it is accompanied by a decrease in revertant frequency counting per one dose of the regulatory element (Koterniak, in press).
Such a non-linear dependence between the reversion frequency and the dose of Bg-hf resembles to a certain degree the dosage effects observed for the Ac-Ds system of transposable elements, where an increase in the Ac dose leads to a decrease in somatic mutations at the Ac controlled loci. For the Ac-Ds interaction it is supposed that the maximum excision frequency is observed under a certain optimal level of the Ac encoded transposase, deviation from which leads to a decrease in excision frequency of the receptor element Ds. It is assumed that the inhibition of excisions under the high transposase concentration may be connected with the aggregation of transposase molecules and the regulation of transposase activity on its substrate site (see reviews in Heinlein, Genetics 144:1851-1869, 1996; Brutnell, Genetics 147:823-834, 1997). Taking into account sequence similarity between Ac and Bg (Hartings, Mol. Gen. Genet. 197:209-218, 1984), it is possible to suppose that the mentioned decrease in excision frequency of the receptor element rbg also could be connected with a certain kind of association of the Bg encoded transposase.
Activity in ontogeny. One of the characteristic features of the Bg-rbg system of transposable elements, observed under the control of expression of the mutable alleles at the o2 locus, is the fact that excisions of the receptor element rbg from this locus take place at postmeiotic mitotic divisions during mega- or microsporogenesis or during endosperm development, but not during embryo or sporophyte development Montanelli, Mol. Gen. Genet. 227:91-96, 1984).
However, it is well known that the activity of regulatory elements (e. g. Ac) can show high developmental and tissue specificity. Such properties could be connected both with their DNA sequences and the positional effect of transposable elements (Peterson, Mol. Gen. Genet. 149:5-21, 1976; Fedoroff, pp. 1-63 in Mobile Genetic Elements, New York: Academic Press, 1983). Data available allow concluding that the specificity of the Bg-rbg system in relation to the mutable alleles at the o2 locus also may be connected with the properties of the Bg regulatory element sequences and with the positional effect of the Bg and rbg insertions.
Thus, the study of a Bg-induced allele at the wx1 locus
showed that the Bg activity is not restricted to the endosperm tissue
(Motto, Maydica 34:107-122, 1989). Besides, in the studies of the Bg-controlled
receptive alleles at the o2 locus it was established that the reversion
of the o2-hf allele can occur in the late stages of the ear development
before meiosis (Koterniak, MNL 76:54, 2002).
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