Penicillin tolerance genes of Streptococcus pneumoniae: the ABC-type manganese permease complex Psa

Downregulation of the major autolysin in Streptococcus pneumoniae leads to penicillin tolerance, a feature that is characterized by the ability to survive but not grow in the presence of antibiotic. Screening a library of mutants in pneumococcal surface proteins for the ability to survive 10x minimum inhibitory concentration (MIC) of penicillin revealed over 10 candidate tolerance genes. One such mutant contained an insertion in the known gene psaA, which is part of the psa locus. This locus encodes an ABC-type Mn permease complex. Sequence analysis of adjacent DNA extended the known genetic organization of the locus to include two new open reading frames (ORFs), psaB, which encodes an ATP-binding protein, and psaC, which encodes a hydrophobic transmembrane protein. Mutagenesis of psaB, psaC, psaA and downstream psaD resulted in penicillin tolerance. Defective adhesion and reduced transformation efficiency, as reported previously for a psaA- mutant, were phenotypes shared by psaB-, psaC- and psaD- knockout mutants. Western blot analysis demonstrated that the set of mutants expressed RecA, but none of them showed translation of the autolysin gene, which is located downstream of recA. The addition of manganese (Mn) failed to correct the abnormal physiology. These results suggest that this ABC-type Mn permease complex has a pleiotropic effect on pneumococcal physiology including adherence and autolysis. These are the first genes suggested as being involved in triggering autolysin. The results raise the possibility that loss of function of PsaA, by vaccine-induced antibody for instance, may promote penicillin tolerance.