## 4 Discussion  
The analysis performed on the promoter regions of _MECPS_ genes from several model plants indicated repetitions of the RSEe consensus sequence in the 5\({}^{\prime}\)UTR regions of two _Zea maysMECPS_ genes, _Zm00001d051458_ and _Zm00001d017608_. The obtained results suggest that the putative dimerization site of CAMTA _trans_-factors, mediated by closely spaced repeated RSEs, is not specific to _S. milltiorrhiza_ but is also observed in other plants. It is interesting that in both plants, the repeated RSEs are localized within the 5\({}^{\prime}\) UTR. Putatively, the stable localization of repeated RSEs within this particular gene regulatory region may depend on potential interactions of CAMTA _trans_-factors with other regulatory proteins present in the Mediator complex [20]. The putative deciding circuit includes the auto-regulation of _MECPS_ gene expression by the product of MECPS enzyme activity, MECPD, which controls the function of CAMTA3 _trans_-factor [29; 32]. The final activation of CAMTA3 _trans_-factor requires its interaction with CAMTA 2 or CAMTA 4, which could be supported by the closely spaced, repeated RSEs within the 5\({}^{\prime}\)UTR of the _MECPS_ gene. The occurrence of such putative auto-regulation in two plant species, _Z. mays_ and _S. milltiorrhiza_, could be related to the particular metabolic conditions of both plants. _S. milltiorrhiza_ has significantly developed secondary metabolic routes for the production ofiterpene derivatives known as tanshinones [1; 4; 5]. These components play an important role in adapting plants to biotic and abiotic stress conditions, and their biosynthesis mainly requires intermediates provided by the MEP pathway [8]. Such specialized metabolic requirements could involve additional regulatory events controlling the expression of the _MECPS_ gene. Though the control of gene transcription is recognized as an approximate adjustment to the metabolic requirements of plant cells, it adds an important regulatory layer to metabolic regulation. The putative positive regulatory feedback between MECPD concentration and _MECPS_ gene expression could help extend the relatively high transcript level of _MECPS_ during long and sunny days [12; 13; 14; 15; 16]. Previous studies suggest that MECPD regulates plant responses to stress that are mediated by oxidative stress induced by UV, elevated temperature, and incorrect protein folding [27; 48]. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Nr** & **Sequence of Closely Spaced RSEe Repetitions** & **Gene ID/Name** \\ \hline
1 & **GCGCGCCTGGCGCGC** & _Zm00001d012197\_T001/CMS_ \\ \hline
2 & **GCGCGCGCGTGCGCGC** & _Zm00001d012197\_T005/CMS_ \\ \hline
3 & **GCGCGCGCGCGCGCGC** & _Zm00001d012197\_T006/CMS_ \\ \hline
4 & **GCGCGCGCGCGCGCGCGCGC** & _Zm00001d012197\_T007/CMS_ \\ \hline
5 & **CCGCGCGCGCAGAGCTGCGCGCGCGC** & _Os03t0732000-00/HDR_ \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Sequences of closely spaced RSE repetitions found in _Zea maysCMS_ gene. Sequences of RSEs are written in bold. A repeated RSEe sequence was found also in the _Oryza sativa HDR_ gene. The distance between both RSEs in _OsHDR_ is higher than the threshold of 8 nt [34]. Therefore, the putative induction of CAMTA dimerization should be interpreted cautiously. Zea mays_ belongs to plants performing C4 photosynthesis, which is more efficient than the common C3 carbon fixation route [49]. Assuming that the two precursor molecules of the MEP pathway (D-glyceraldehyde 3-phosphate and pyruvate) are derived directly from photosynthesis or glycolysis, it could be suggested that an additional regulatory circuit controlling _MECPS_ gene expression is capable of more precise adjustment of sugar metabolism in plants [13]. It is known that sugar (sucrose) concentration regulates the transcription rate of several MEP pathway genes in the dark (_DXS_, _DXR_, _CMK_, _CMS_, and _HDR_) [13; 14].