###### Abstract  
Previous research has demonstrated the presence of two closely spaced repetitions of the rapid stress-responsive _cis_-active element RSRE (G/A/C)CGCG(C/G/T) in the 5 UTR of _S. milliorthiza2C-methyl-D-erithrytol 2,4-cyclodiphosphate synthase_ (_MECPS_) gene. The product of MECPS activity, represented by 2C-methyl-D-erithrytol 2,4-cyclodiphosphate (MECPD), indicates its retrograde regulatory role and activates CAMTA _trans_-factors. Since the complete activation of CAMTA _trans_-factors requires the cooperative interaction of CAMTA3 with CAMTA2 or CAMTA4, the closely spaced RSREs recognized by CAMTA _trans_-factors could be used to promote CAMTA _trans_-factor dimerization. The present study aims to evaluate if the occurrence of these two closely spaced RSREs in the 5 UTR is specific to _S. milliorthiza_ or could be observed in other _MECPS_ genes. An analysis of nineteen _MECPS_ gene sequences from seven selected model plants indicated the closely spaced repetition of RSREs in the 5 UTR region of two maize (_Zea mays_) _MECPS_ genes, _Zm00001d051458_ and _Zm00001d017608_. This observation suggests the potential autoregulatory function of MECPD in relation to the _MECPS_ transcription rate. Moreover, an analysis of eighty-five promoter regions of other plastid methyl-D-erythth! phosphate (MEP) pathway genes indicated such closely spaced RSREs in the proximal promoter of _Zea mays_-C-methyl-D-erylrtidt 4-phosphate cytidyltransferase (CMS) (_Zm00001d012197_) and _Oryza sativa4-hydroxy-3-methylbut-2-enyl diphosphate reductase (HDR)_ (Os030t0732000-00). rapid stress-responsive element; CAMTA dimerization; plant MEP pathway regulation 2023 13 2023 20  
expression regulation, is facilitated by numerous signaling routes, which adjust the gene expression level to the particular needs of cells [18; 19]. External signals are delivered by activated gene-specific _trans_-factors to the multisubunit Mediator complex, interacting with general _trans_-factors complexed with RNA polymerase II [20]. Although transcription plays an important role in regulating pathway activity, it is generally used to roughly adjust the pre-mRNA level to the needs of plant cells [10]. The much more complex mechanisms control the enzyme level and its activity [21]. These regulations are particularly apparent in relation to enzymes, indicating a control function in the metabolite flux of the pathway. Such a role in the MEP route is played by 1-deoxy-D-xylulose-5-phosphate synthase (DXS),which has the highest flux control coefficient in the entire pathway [22]. The activity of DXS is allosterically inhibited by DMAPP and IPP that are produced by the MEP trail. However, when the concentration of IPP and DMAPP decreases, the abundance of DXS protein increases [23; 24]. The putative mechanism linking the inhibition of DXS in the presence of IPP and DMAPP is the increased proteolytic sensitivity of enzyme inactive forms [25]. Another MEP pathway enzyme with a critical role is 2C methyl-D-erithyrtol 2,4-cyclodiphosphate synthase (MECPS) (EC: 4.6.1.12), which catalyzes the conversion of 4-(cytidine 5'diphospho)-2C-methyl-D-erithyrtol 2-phosphate into 2C-methyl-D-erithyrtol 2,4-cyclodiphosphate (MECPD) [9]. The product of MECPS activity represents the retrograde signaling molecule that regulates the expression of nuclear genes [26; 27; 28; 29]. A common property of genes controlled by MECPD is the overrepresentation of _cis_-active motifs (G/A/C)CGCG(C/G/T), which are known as rapid stress response elements (RSREs) [30; 31; 32]. The gene expression regulation by RSREs requires binding to calmodulin-binding transcription activator 3 (CAMTA3). Moreover, CAMTA3 cooperatively interacts with CAMTA2 or CAMTA4 [32]. The homo- or hetero-dimerization of _trans_-factors triggers a sequence of regulatory events that lead to a particular cellular fate, increasing the precision and complexity of gene expression regulation [33].