Penicillin binding proteins (PBPs) catalyze essential steps in the biosynthesis of peptidoglycan, the main component of the bacterial cell wall. PBPs can harbor two catalytic domains, namely the glycosyltransferase (CT) and transpeptidase (TP) activities, the latter being the target for beta-lactam antibiotics. Despite the availability of structural information regarding bi-functional PBPs, little is known regarding the interaction and flexibility between the TP and CT domains. Here, we describe the structural characterization in solution by small angle X-ray scattering (SAXS) of PBP1b, a bi-functional PBP from Streptococcus pneumoniae. The molecule is present in solution as an elongated monomer. Refinement of internal coordinates starting from a homology model yields models in which the two domains are in an extended conformation without any mutual contact compatible with the existence of restricted mobility. (c) 2011 Elsevier Inc. All rights reserved.