## 2 Results
### Transcriptional Regulators and Sensory Histidine Kinases
#### 2.2.5 Cluster E: Regulatory Proteins More Abundant in Pi Limitation than in Pi proficiency in SC  
Cluster E can be divided in two sub-clusters: E1 and E2 (Figure 6). The 23 regulators of sub-cluster E1 were clearly more abundant in Pi limitation than in Pi proficiency in SC but not in SL. Sub-cluster E1 includes:  
* SCO/327/OsaD, thought to be involved in OsS sensing [66,126]. * The zinc-binding regulator HypR/SCO6294 [127], whose expression is induced by hydroxproline. SCO6294 controls the expression of the Hyp operon (_sco6289-sco6293_) involved with L-hydroxproline catabolism. Since hydroxproline is known to enhance the activity of the peptidoglycan N-deacetylase [128], the induction of HypR expression in SC in Pi limitation might be related to cell wall alteration in SC. This cluster also includes several regulators playing in role in the regulation of antibiotic biosynthesis and/or morphological differentiation, such as:  
* The RR DraR/SCO3063 of the TCS DraK (sub-cluster A2)/DraR, which has a positive impact on ACT biosynthesis but a negative one on CPK and RED biosynthesis [62]. * The ArsR-like regulator SCO6808, which has a negative impact on antibiotic biosynthesis [117]  
Figure 6: Cluster E, heatmap representation of transcriptional regulators and sensory histidine kinases more abundant in Pi limitation than in Pi proficiency in SC. Cluster E can be divided into 2 sub-clusters, E1 and E2. The 23 regulators of sub-cluster E1 were more abundant in Pi limitation than in Pi proficiency in SC but not in SL. The 7 regulators of sub-cluster E2 were more abundant in Pi limitation than in Pi proficiency in SC but were also abundant at 60 h in Pi proficiency in SL. The sensory HK CutS/SCO5863 of the TCS CutR (cluster C1)/CutS, which is thought to be involved in the regulation of copper homeostasis [105] and has a negative impact on RED and ACT biosynthesis [106]. - The KorSA-like regulator SCO3932 is located upstream of a pSAM2-like ICE element and has a positive impact CPK and ACT synthesis via its direct interaction with _cpkD_, a coelimycin biosynthetic gene, and _actll-orf4_[129]. - The RR SCO5785 of the TCS SCO5784/SCO5785 has a positive impact on the production of several extracellular proteins and on antibiotics biosynthesis, and a negative one on the expression of some ribosomal genes [130]. - The MmyB-like regulator SCO4944 is a putative member of the A-factor signaling cascade [131] and is likely to have a positive impact on antibiotic biosynthesis. - The protein WhiA/SCO1950 which governs spore formation [132,133]. - The 7 regulators of sub-cluster E2 were also more abundant in Pi limitation than in Pi proficiency in _SC_ but were also abundant at 60 h in Pi proficiency in _SL_. This cluster includes _sco3205_ encoding a regulator of unknown function [134,135], as well as _vanR/sco3590_, encoding an activator of the expression of the divergently located _vanJKHAX_ cluster conferring inducible vancomycin resistance [136,137]. The low abundance of SCO3590 in Pi proficiency in _SC_ could be seen as consistent with the reported hyper-sensitivity of _SC_ to vancomycin in Pi proficiency [138]. The VanRS and CseBC TCS (sub-cluster B1) are both involved in the signaling of peptidoglycan/cell wall alteration [136,137,139]. - 2.2.6. Cluster F: Regulators More Abundant in _SL_ than in _SC_ in Both Pi Conditions - Cluster F includes 44 regulatory proteins more abundant in _SL_ than in _SC_ in both Pi conditions (Figure 7). This cluster includes regulators known to have either a positive or a negative impact on antibiotic biosynthesis and/or morphological biosynthesis in _SC_ and some of them are involved in the regulation of metal homeostasis.