EloR Interacts with the Lytic Transglycosylase MltG at Midcell in Streptococcus pneumoniae R6

The ellipsoid shape of Streptococcus pneumoniae is determined by the synchronized actions of the elongasome and the divisome, which have the task of creating a protective layer of peptidoglycan (PG) enveloping the cell membrane. The elongasome is necessary for expanding PG in the longitudinal direction, whereas the divisome synthesizes the PG that divides one cell into two. Although there is still little knowledge about how these two modes of PG synthesis are coordinated, it was recently discovered that two RNA-binding proteins called EloR and KhpA are part of a novel regulatory pathway controlling elongation in S. pneumoniae. EloR and KhpA form a complex that works closely with the Ser/Thr kinase StkP to regulate cell elongation. Here, we have further explored how this regulation occurs. EloR/KhpA is found at midcell, a localization fully dependent on EloR. Using a bacterial two-hybrid assay, we probed EloR against several elongasome proteins and found an interaction with the lytic transglycosylase homolog MltG. By using EloR as bait in immunoprecipitation assays, MltG was pulled down, confirming that they are part of the same protein complex. Fluorescence microscopy demonstrated that the Jag domain of EloR is essential for EloR's midcell localization and its interaction with MltG. Since MltG is found at midcell independent of EloR, our results suggest that MltG is responsible for the recruitment of the EloR/KhpA complex to the division zone to regulate cell elongation. IMPORTANCE Bacterial cell division has been a successful target for antimicrobial agents for decades. How different pathogens regulate cell division is, however, poorly understood. To fully exploit the potential for future antibiotics targeting cell division, we need to understand the details of how the bacteria regulate and construct the cell wall during this process. Here, we have revealed that the newly identified EloR/KhpA complex, regulating cell elongation in S. pneumoniae, forms a complex with the essential peptidoglycan transglycosylase MltG at midcell. EloR, KhpA, and MltG are conserved among many bacterial species, and the EloR/KhpA/MltG regulatory pathway is most likely a common mechanism employed by many Gram-positive bacteria to coordinate cell elongation and septation.