An embarrassment of sortases – a richness of substrates? Response

Abstract

Pallen and colleagues report that the genomes of many Gram-positive bacteria harbour more than one sortase gene. The multiplicity of sortase genes is indeed astonishing. Streptomyces coelicolor is the winner with no less than seven sortase genes, whereas Bacillus subtilis has two but only one gene encoding a sorting signal (the substrate for sortase) 1. What can be the role for the multiplicity of sortase or sortase-like genes? Pallen and colleagues suspect redundancy of function could be one possibility. This seems somewhat remote to us, as true redundancy is rarely observed in bacterial genomes. One example of redundancy is the bacterial ribosomal RNA genes (rrn). Escherichia coli for example synthesizes proteins with seven rrn genes 2,3 – each can be knocked out without loss of viability on agar plates in the laboratory 4. It is conceivable however, that the rrn genes might build ribosomes with dedicated functions 5. Another example of genetic redundancy is the penicillin-binding-protein (PBP) family. We know the genes encoding these proteins are essential for bacterial growth and cell wall synthesis as their inhibition with β-lactam antibiotics results in microbial killing 6. However, knockout mutations of many pbp genes do not affect bacterial growth on agar plates 7. Nevertheless, it appears that each PBP synthesizes unique aspects of the murein sacculus that might be essential for bacterial growth outside of laboratories 8.