Structure of the neuraminidase locus in different streptococci. A. The schematic maps of the nanAB operon of S. pneumoniae G54 and the orthologous locus in its close relatives, including S. gordonii V288 (NC_009785.1), S. sanguinis SK36 (NC_009009.1), S. mitis B6 (NC_013853.1) and S. oralis Uo5 (NC_015291.1) are shown. In S. pneumoniae the complete locus includes 18 ORFs, some of them conserved in the other species. The two neuraminidases (NanA and NanB) are in pink, while the three different transporters (two ABC transporters and one PTS) are in blue. The phosphosugar binding transcriptional regulator is shown in grey and the metabolic enzymes involved in sialic acid metabolism are in orange. The homologous regions in green refer to DNA identity above 50% and represent orthology of genes. The black arrows placed upstream of SPG1601, SPG1599, SPG1593, and SPG1583 represent the promoters of the regulon. The gene numeration is detailed in Table1. B. Schematic representation of the first steps in sialic acid catabolism. The first step involves the N-acetylneuraminate lyase SPG1585 which removes a pyruvate group from sialic acid, yielding N-acetylmannosamine (ManNAc). Subsequently, an N-acetylmannosamine kinase (SPG1584) adds a phosphate group to ManNAc, resulting in the formation of N-acetylmannosamine-6-phosphate (ManNAc-6P). SPG1593 encodes an N-acetylmannosamine-6-phosphate 2-epimerase, which transforms ManNAc-6P into N-acetylglucosamine-6-phosphate (GlcNAc-6P)[15, 16].