Reversible phosphorylation of bacterial transcriptional regulators (TRs) belonging to the family of two-component systems (TCSs) is a well-established mechanism for regulating gene expression. Recent evidence points to the fact that reversible phosphorylation of bacterial TRs on other types of residue, i.e. serine, threonine, tyrosine and cysteine, is also quite common. The phosphorylation of the ester type (phospho-serine/threonine/tyrosine) is more stable than the aspartate phosphorylation of TCSs. The kinases which catalyse these phosphorylation events (Hanks-type serine/threonine protein kinases and bacterial protein tyrosine kinases) are also much more promiscuous than the TCS kinases, i.e. each of them can phosphorylate several substrate proteins. As a consequence, the dynamics and topology of the signal transduction networks depending on these kinases differ significantly from the TCSs. Here, we present an overview of different classes of bacterial TR phosphorylated and regulated by serine/threonine and tyrosine kinases. Particular attention is given to examples when serine/threonine and tyrosine kinases interact with TCSs, phosphorylating either the histidine kinases or the response regulators. We argue that these promiscuous kinases connect several signal transduction pathways and serve the role of signal integration. Bacterial two-component systems (TCSs)TCSs are signal transduction devices that were initially discovered in bacteria (Ninfa & Magasani, 1986;Nixon et al., 1986). They play an important role in signal sensing and response to various stimuli, enabling the organisms to adapt to environmental changes. A typical TCS consists of a histidine kinase (HK) and a corresponding response regulator (RR) (Stock et al., 2000;Gao & Stock, 2009). HK usually possesses a highly variable sensor domain and a conserved kinase core. Following environmental stimulus, a signal ligand binds to the sensor domain and results in the autophosphorylation of the kinase core at a conserved histidine residue, at the expense of ATP. Next, the phosphoryl group is transferred from HK to a conserved aspartate in the regulatory domain of the RR. RRs usually contain two domains: a regulatory domain with the conserved phosphorylatable aspartate and a variable effector domain. Phosphorylation activates the effector domain of RRs, triggering the physiological response. As phosphohistidine has a very short half-life in aqueous solutions at neutral pH, in the absence of the environmental signal the system switches itself off very rapidly.Many effector domains of bacterial RRs have DNA-binding capacity. This allows RRs to function as transcriptional regulators (TRs) and consequently change gene transcription when they become phosphorylated. In Escherichia coli, osmoregulation of porin proteins OmpF and OmpC is under transcriptional control of the TCS EnvZ/OmpR. The phosphorylation of OmpR by EnvZ changes its affinity for the promoter region of ompF and ompC, resulting in different transcriptional levels of these genes (Forst et al., 1...

3-hydroxypropanoic acid (3-HP) is a valuable platform chemical with a high demand in the global market. 3-HP can be produced from various renewable resources. It is used as a precursor in industrial production of a number of chemicals, such as acrylic acid and its many derivatives. In its polymerized form, 3-HP can be used in bioplastic production. Several microbes naturally possess the biosynthetic pathways for production of 3-HP, and a number of these pathways have been introduced in some widely used cell factories, such as Escherichia coli and Saccharomyces cerevisiae . Latest advances in the field of metabolic engineering and synthetic biology have led to more efficient methods for bio-production of 3-HP. These include new approaches for introducing heterologous pathways, precise control of gene expression, rational enzyme engineering, redirecting the carbon flux based on in silico predictions using genome scale metabolic models, as well as optimizing fermentation conditions. Despite the fact that the production of 3-HP has been extensively explored in established industrially relevant cell factories, the current production processes have not yet reached the levels required for industrial exploitation. In this review, we explore the state of the art in 3-HP bio-production, comparing the yields and titers achieved in different microbial cell factories and we discuss possible methodologies that could make the final step toward industrially relevant cell factories.

3-Hydroxypropanoic acid (3-HP) is an important biomass-derivable platform chemical that can be converted into a number of industrially relevant compounds. There have been several attempts to produce 3-HP from renewable sources in cell factories, focusing mainly on Escherichia coli, Klebsiella pneumoniae, and Saccharomyces cerevisiae. Despite the significant progress made in this field, commercially exploitable large-scale production of 3-HP in microbial strains has still not been achieved. In this study, we investigated the potential of Bacillus subtilis as a microbial platform for bioconversion of glycerol into 3-HP. Our recombinant B. subtilis strains overexpress the two-step heterologous pathway containing glycerol dehydratase and aldehyde dehydrogenase from K. pneumoniae. Genetic engineering, driven by in silico optimization, and optimization of cultivation conditions resulted in a 3-HP titer of 10 g/L, in a standard batch cultivation. Our findings provide the first report of successful introduction of the biosynthetic pathway for conversion of glycerol into 3-HP in B. subtilis. With this relatively high titer in batch, and the robustness of B. subtilis in high density fermentation conditions, we expect that our production strains may constitute a solid basis for commercial production of 3-HP.