Insights into the evolutionary relationships of lyta autolysin and ply pneumolysin-LikeGenes in streptococcuspneumoniae and related streptococci

Abstract

Streptococcus pneumoniae (pneumococcus) is amajor human pathogen. Themain pneumococcal autolysin LytA and the pneumolysin Ply are two of the bacterium's most important virulence factors. The lytA- And ply-related genes are also found in other streptococci of the Mitis group (SMG). The precise characteristics of the lytA-related-but not the ply-related-genes of SMG and theirprophages havebeenpreviously described.Asearch of themore than400SMGgenomicsequences available inpublicdatabases (ca. 300 for S. pneumoniae), showed Streptococcus pseudopneumoniae IS7493 to harbor four ply-related genes, two of which (plyA and plyB) have 98%identical nucleotides. The plyA homolog of S. pseudopneumoniae is conserved in all S. pneumoniae strains, and seems to be included in a pathogenicity island together with the lytA gene. However, only nonencapsulated S. pneumoniae strains possess a plyB gene,which is part of an integrative and conjugative element. Notably, the existence of a bacterial lytA-related gene in a genome is linked to the presence of plyA and vice versa. The present analysis also shows there are eight main types of plyA lytA genomic islands. A possible stepwise scenario for the evolution of the plyA lytA island in S. pneumoniae is proposed.