Pascani, Republic of Moldova
Maize and Sorghum Research Institute
Transposable genetic elements as factors establishing novel regulatory links: models explaining experimental data --Koterniak VV We have reported about closely related lines derived from disruptive selection for frequency of reversion of a mutable allele at the opaque2 (o2) locus, that is controlled by the Bg-rbg system of transposable elements. Sharp differences exist among these lines with respect to properties of the components of the Bg-rbg system of transposable elements and the expression of quantitative traits (Maydica 44:195-203, 1999; MNL 73:76-79; 74:57-58; this issue), suggesting a connection between the action of transposable elements and the expressivity of quantitative traits. Possible mechanisms explaining such a connection are discussed below.

Model 1. Taking into account that o2 is an important regulatory gene in zein accumulation, we propose that o2 regulates the quantitative traits under consideration (especially the ones related to kernel weight and volume). For these traits, changes in o2 activity (conditioned for example by changes in state of a mutable allele) would modify their expressivity (Fig. 1).

Model 2. Changes (e. g. internal deletions) in the receptor elements contained in the mutable o2 alleles can affect expression of other genes through transposition of the receptor elements into these genes (Fig. 2). Taking into account differences between the receptor elements contained in the o2-lf and o2-hf alleles of lines studied (Maydica 44:195-203, 1999; MNL, this issue), we can expect that transpositions of the receptor elements in the same genes will lead to different levels of expression of these genes.

Other Models. Other schemes for regulatory action of transposable elements invoke gene products of transposable elements. In Cuypers et al. (EMBO J. 7:2953-2960, 1988) it was reported that a defective En-I102 element reduced the excision frequency of both the autonomous En-1 element and the inhibitor element Spm-I5719A. The authors suggested that the changed product of a defective En-I102 element acted as a competitive inhibitor and was responsible for reduced excisions.

Because differences in quantitative traits between lines studied did not depend on the activity of the regulatory (autonomous) element (MNL, 74:57-58; this issue), we can assume that the interactions between receptor elements play a predominant role. If the receptor element inserted into o2 encodes the a competitive inhibitor of the products encoded by the rbg element inserted into quantitative trait genes, can expect changes in expressivity of quantitative traits (Fig. 3). In this model insertion of the receptor element into o2 would show pleiotropic action regarding quantitative trait loci (which could be observed as pleiotropic action of the o2 gene). In addition, in this model we could observe the appearance of new regulatory links between previously independent genes.

Furthermore, it is possible that regulatory activities of o2 may be subject to a "feedback" type of regulation , based on the competitive interaction of different products of rbg (Fig. 4).

Earlier we reported the possibility of rapid, inheritable changes in traits under transposable element control (Maydica 44:195-203, 1999; MNL 73:76-79). Study of the expressivity of quantitative traits in the lines with differing states of Bg-rbg components suggests that transposable elements can be responsible for significant modification and determination of the new regulatory links between genes, invoking the involvement of transposable elements in evolution.

Figure 1. Interaction between (1) the regulatory gene o2, carrying insertion of a rbg element (designated as rbg-s1 (state 1), represented by a horizontally hatched box on the figure) and (2) a quantitative trait gene, designated as genX, which determines trait X. Expression of genX is regulated by o2. A change in state in the rbg-s1 element (e.g. due to internal deletion in this element) leads to the appearance of the rbg-s2 element (represented by vertically hatched box). This change alters activity of o2, which in turn leads to modified expression of trait X (the modified trait is designated as Xmod).

Figure 2. Receptor element rbg-s2 (derived from an internal deletion in rbg-s1 on o2) can transpose to a certain gene, genY, which is not regulated by o2. genY participates in determination of trait Y. Transposition of the rbg-s2 element changes activity of the genY gene and modifies trait Y (designated as Ymod).

Figure 3. The interaction with rbg products. The rbg-s2 (vertically hatched square) and the rbg-s1 product are competitive inhibitors (horizontally hatched circle). The rbg-s2 arose by internal deletion, from rbg-s1 contained in the o2 gene. genZ expression is conditioned by interactions between the rbg-s1 insert and the rbg-s2 product. Though o2 does not normally regulate, the expression of genZ is now affected by the product of the rbg-s2 element contained within o2. Thus changes in state of o2 can lead to changes of expression not only of the o2 gene, but also the genZ.

Figure 4. The "feedback" type of interaction based on rbg products. The product of the rbg-s2 (vertically hatched square) acts as competitive inhibitor of the rbg-s1 product (horizontally hatched circle). The rbg-s2 insert arose by an internal deletion of the rbg-s1 present in genX. Activity of o2 is conditioned by the interaction between the rbg-s1 insert present in this gene and the product of the rbg-s2 element. Expression of the genX gene remains regulated by o2, but now the product of the rbg-s2 insert into genX modifies the activity of o2.
 
 


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