## 1 Introduction  
Actinomycetes, and especially _Streptomyces_, are industrially and medically important Gram-positive bacteria responsible for the production of the majority of antibiotics in current use [1]. Nutritional limitations in phosphate (Pi) and/or nitrogen (N) are thought to trigger a complex morphological differentiation process that usually coincides with the production of specialized bio-active metabolites [2, 3]. However, the apprehension of the nature of the extra- and/or intra-cellular signals [4] and of the role of the global and specific regulators, and their interplay [5], in the triggering of the biosynthesis of specialized metabolites is still incomplete, even after half a century of active research. _Streptomyces coelicolor_ (_SC_) [6] and _Streptomyces_ _lividans_ (_SL_) [7] are the model strains extensively used to study the regulation of specialized metabolites biosynthesis since these closely related strains possess the same biosynthetic pathways directing the biosynthesis of three well-characterized bio-active specialized metabolites, calcium-dependent antibiotic (CDA), undecylprodigiosion (RED), and actinorhodin (ACT). However, these metabolites are mainly produced by _SC_[8] and, consistently, proteins encoded by genes of these pathways are far more abundant in _SC_ than in _SL_[9]. In order to gain a better understanding of the different physiological and metabolic features underpinning the contrasted biosynthetic abilities of these two strains to produce antibiotics and of the impact of phosphate availability on the latter, an in-depth comparative analysis of the proteome of these two strains grown for 48 and 60 h on the solid modified R2 YE medium either limited (1 mM) or proficient in Pi (5 mM), was carried out. Relative abundance and regulatory features of proteins of the primary and specialized metabolisms of these strains were previously published in [PERSON] et al. [9]. In this issue we present a comparative analysis of the abundance of proteins of the transcriptional apparatus and regulatory proteins of one and two component systems, as well as eukaryotic-like protein kinases of these two strains. This report is based on the same set of proteomic data as [PERSON] et al. [9], so efforts were made throughout the present manuscript to establish correlations between the abundance of mentioned regulators and that of their established or putative regulatory targets via the systematic quotation of the figure numbers of [PERSON] et al. [9]. The study of [PERSON] et al. [9] revealed major differences between \(SC\) and \(SL\). One was the far higher abundance of subunits of the complex I/NADH dehydrogenase of the respiratory chain, (nuoABCDEFGHIJKLMN/SCO4562-75) in \(SC\) compared to \(SL\) that might contribute to the reported high ATP content of this strain [10] as well as to the generation of ROS/NOS [11] leading to oxidative stress (OxS), a proposed important trigger of ACT biosynthesis [8; 12]. The other was the poor abundance of proteins involved in nitrogen up-take and assimilation in \(SC\) compared to \(SL\), likely to result in severe nitrogen stress. Since nitrogen (N), carbon (C), and phosphate (P) assimilation ought to be regulated in _Streptomyces_[13], as in all living organisms [14], the poor ability of \(SC\) to assimilate N is correlated with the reduced abundance of proteins playing a role in C up-take and assimilation such as the glucose permease (SCO5578) (Figure 16B of [9]) and all glycolytic enzymes (Figure 2 of [9]) in \(SC\) compared to \(SL\). Similarly, proteins of the Pho regulon involved in Pi up-take and assimilation were less abundant in \(SC\) than in \(SL\) but not to the same extent as those of N and C metabolisms [8].