The sus and scr systems are regulated by LacI family repressors SusR and ScrR respectively. SusR regulates an adjacent ABC transporter (susT1/susT2/susX) and sucrose-6-phosphate (S-6-P) hydrolase (susH). ScrR controls an adjacent PTS transporter (scrT), fructokinase (scrK) and second S-6-P hydrolase (scrH). sus and scr play niche-specific roles in virulence.
The susH and sus locus mutants are attenuated in the lung, but dispensable in nasopharyngeal carriage. Conversely, the scrH and scr locus mutants, while dispensable in the lung, are attenuated for nasopharyngeal colonization.
The scrH/susH double mutant is more attenuated than scrH in the nasopharynx, indicating SusH can substitute in this niche.
Both systems are sucrose-inducible, with ScrH being the major in vitro hydrolase.
The scrH/susH mutant does not grow on sucrose indicating that sus and scr are the only sucrose-metabolizing systems in S. pneumoniae.
We propose a model describing hierarchical regulation of the scr and sus systems by the putative inducer, S-6-P. The transport and metabolism of sucrose or a related disaccharide thus contributes to S. pneumoniae colonization and disease.