## 1 Introduction
Such orthogonal biosensors are supposed to interact with the endogenous cellular network less commonly, which might be favorable for scoring production [19], but might be a disadvantage when native regulatory mechanisms are examined. Here, a biosensor based on the metabolite-dependent MarR-type transcriptional repressor CrtR from _C. glutamicum_ is described. This soil bacterium with GRAS status has been used safely in industrial amino acids production for over 60 years [20] since its discovery as a natural glutamate producer [21]. _C. glutamicum_ has been metabolically engineered for the sustainable production of various, mostly nitrogenous, compounds [22]. Notably, _C. glutamicum_ is a natural carotenoid producer, and its yellow pigmentation is due to the unusual C50 carotenoid decaprenoxanthin [23,24]. The carotenoid precursor GGPP is synthesized from IPP and DMAPP (dimethylallyl pyrophosphate), which are generated from pyruvate and GAP in the MEP pathway [23, 24, 25], primarily by the prenyltransferase IdAs [26]. The genes that are necessary for the conversion of GGPP to the final yellow decaprenoxanthin are organized in a single operon (_crtE_, _cg0722_, _crtB_, _crtI_, _crtYc_, _crtYf_, _crtEb_) transcribed from _PcrtE_[24]. The knowledge about carotenogenesis in _C. glutamicum_ has guided metabolic engineering in such a way as to enhance production of the native decaprenoxanthin [23], and to enable the production of nonnative C40 and C50 carotenoids [27; 28; 29], including the industrially relevant astaxanthin [30; 31]. Several metabolic engineering approaches have been used to improve the production of terpenoids by _C. glutamicum_. First, _dxs_ encoding the committed enzyme in the MEP pathway was overexpressed [32; 33]. Second, balancing the DMAPP to IPP ratio via the overexpression of _idi_ improved patchoulol production when combined with _dxs_ overexpression [8]. Third, the overproduction of the two endogenous GGPP synthases IdsA and CrtE enhanced decaprenoxanthin production due to increased synthesis of GGPP [26]. The overexpression of these genes from IPTG-inducible promoters was orthogonal and independent from endogenous transcription regulatory feedback. Recently, a membrane-fusion protein comprising CrtZ and CrtW was published, and it was shown that the additional overexpression of precursor biosynthesis genes enhanced astaxanthin product formation [31]. In this regard, precursor-dependent transcriptional regulation may be beneficial, for example in the on-demand conversion of GGPP to the chosen target terpenoid. A biosensor system for the detection of GGPP would represent a powerful tool for strain development, in particular with regard to investigations into the MEP pathway concerning efficient precursor supply. _C. glutamicum_ possesses the transcriptional repressor CrtR for the control of decaprenoxanthin biosynthesis [23]. Like most MarR-type regulators, _C. glutamicum_ CrtR represses gene transcription by binding to the intergenic region between its own gene and the divergently oriented _crt_ operon [23; 34]. In vitro analysis showed that isoprenoid pyrophosphates act as inducers of CrtR in _C. glutamicum_. CrtR binding to _CrtE_ was inhibited by GGPP, and to lesser extents by FPP, GPP, DMAPP and IPP [23]. Thus, in _C. glutamicum_, GGPP leads to the derepression of the _crt_ operon and its own gene by CrtR in a metabolite-dependent feed forward mechanism [23]. CrtR has also been associated with the light-dependent regulation of carotenogenesis, although the mechanism remains unclear [35]. CrtR homologs are found mainly in actinobacteria, and _crtR_ genes often cluster with carotenoid biosynthetic genes and/or a _mmpL_-like transporter gene [23]. This suggested a conserved regulatory function of the CrtR orthologs with respect to transcriptional control of carotenoid biosynthetic genes and/or a _mmpL_-like transporter gene in other actinobacteria. MmpL (mycobacterial membrane protein large) proteins often export hydrophobic or lipid-like substances across the cell membrane in mycobacteria [36].