Structural Insights into of the Allosteric Activation of the LicT Antiterminator by PTS-Mediated Phosphorylation

OUP accepted manuscript

Regulated transcription termination provides an efficient and responsive means to control gene expression. In bacteria, rho-independent termination occurs through the formation of an intrinsic RNA terminator loop, which disrupts the RNA polymerase elongation complex, resulting in its dissociation from the DNA template. Bacteria have a number of pathways for overriding termination, one of which is the formation of mutually exclusive RNA motifs. ANTAR domains are a class of antiterminator that bind and stabilize dual hexaloop RNA motifs within the nascent RNA chain to prevent terminator loop formation. We have determined the structures of the dimeric ANTAR domain protein EutV, from Enterococcus faecialis, in the absence of and in complex with the dual hexaloop RNA target. The structures illustrate conformational changes that occur upon RNA binding and reveal that the molecular interactions between the ANTAR domains and RNA are restricted to a single hexaloop of the motif. An ANTAR domain dimer must contact each hexaloop of the dual hexaloop motif individually to prevent termination in eubacteria. Our findings thereby redefine the minimal ANTAR domain binding motif to a single hexaloop and revise the current model for ANTAR-mediated antitermination. These insights will inform and facilitate the discovery of novel ANTAR domain RNA targets.

Resolving the activation mechanism of the D99N antiterminator LicT protein

LicT is an antiterminator protein of the BglG family whose members are key players in the control of carbohydrate catabolism in bacteria. These antiterminators are generally composed of three modules, an N-terminal RNA-binding domain (CAT) followed by two homologous regulation modules (PRD1 and PRD2) that control the RNA binding activity of the effector domain via phosphorylation on conserved histidines. Although several structures of isolated domains of BglG-like proteins have been described, no structure containing CAT and at least one PRD simultaneously has yet been reported in an active state, precluding detailed understanding of signal transduction between modules. To fulfill this gap, we recently reported the complete NMR sequence assignment of a constitutively active mutant (D99N) CAT-PRD1*, which contains the effector domain and the first regulation domain of LicT. As a follow-up, we have determined and report here the 3D solution structure of this active, dimeric LicT construct (40 kDa). The structure reveals how the mutation constrains the PRD1 regulation domain into an active conformation which is transduced to CAT via a network of negatively charged residues belonging to PRD1 dimeric interface and to the linker region. In addition, our data support a model where BglG-type antitermination regulatory modules can only adopt a single conformation compatible with the active structure of the effector domain, regardless of whether activation is mediated by mutation on the first or second PRD. The linker between the effector and regulation modules appears to function as an adaptable hinge tuning the position of the functional modules.

Exploration of key residues and conformational change of anti-terminator protein GlpP for ligand and RNA binding

Anti-terminator protein GlpP regulates gene expression of glycerol uptake operon at post-transcriptional level in a number of bacteria. By now, the molecular dynamics details of ligand and RNA binding by GlpP are still obscure. In this study, we employed the molecular dynamic (MD) simulation and constructed a functional verification platform of GlpP to resolve these puzzles. By combining molecular docking, MD simulation and alanine scanning mutagenesis, a ligand binding pocket consisting of R14, R104 and R157 was identified. Among these residues with positive charge, R14 was dominant for binding glycerol-3-phosphate (G3P). Moreover, the "parallel to crossed" conformational change of the predicted RNA binding region was observed in MD simulation. In this process, the interaction between R104 and E129 was crucial to trigger the conformational change. To further verify this speculation, three ligand independent mutants were obtained by error-prone PCR. The MD simulation indicated that the conformational change happened in all the three mutants, confirming the "parallel to crossed" conformational change endowed GlpP the activity of binding RNA. In recent years, as a potable biological part, anti-terminator was more and more widely used to regulate gene expression in metabolic engineering and synthetic biology. The work in this study deepened our understanding to the typical anti-terminator GlpP, contributing to the further engineering and application of this type of regulator.

The mannose phosphotransferase system (Man-PTS) - Mannose transporter and receptor for bacteriocins and bacteriophages

Mannose transporters constitute a superfamily (Man-PTS) of the Phosphoenolpyruvate Carbohydrate Phosphotransferase System (PTS). The membrane complexes are homotrimers of protomers consisting of two subunits, IIC and IID. The two subunits without recognizable sequence similarity assume the same fold, and in the protomer are structurally related by a two fold pseudosymmetry axis parallel to membrane-plane (Liu et al. (2019) Cell Research 29 680). Two reentrant loops and two transmembrane helices of each subunit together form the N-terminal transport domain. Two three-helix bundles, one of each subunit, form the scaffold domain. The protomer is stabilized by a helix swap between these bundles. The two C-terminal helices of IIC mediate the interprotomer contacts. PTS occur in bacteria and archaea but not in eukaryotes. Man-PTS are abundant in Gram-positive bacteria living on carbohydrate rich mucosal surfaces. A subgroup of IICIID complexes serve as receptors for class IIa bacteriocins and as channel for the penetration of bacteriophage lambda DNA across the inner membrane. Some Man-PTS are associated with host-pathogen and -symbiont processes.