Genomic Island TnSmu2 of Streptococcus mutans Harbors a Nonribosomal Peptide Synthetase-Polyketide Synthase Gene Cluster Responsible for the Biosynthesis of Pigments Involved in Oxygen and H ₂ O ₂ Tolerance

Abstract

The oral biofilm community consists of >800 microbial species, among which Streptococcus mutans is considered a primary pathogen for dental caries. The genomic island TnSmu2 of S. mutans comprises >2% of the genome. In this study, we demonstrate that TnSmu2 harbors a gene cluster encoding nonribosomal peptide synthetases (NRPS), polyketide synthases (PKS), and accessory proteins and regulators involved in nonribosomal peptide (NRP) and polyketide (PK) biosynthesis. Interestingly, the sequences of these genes and their genomic organizations and locations are highly divergent among different S. mutans strains, yet each TnSmu2 region encodes NRPS/PKS and accessory proteins. Mutagenesis of the structural genes and putative regulatory genes in strains UA159, UA140, and MT4653 resulted in colonies that were devoid of their yellow pigmentation (for strains UA140 and MT4653). In addition, these mutant strains also displayed retarded growth under aerobic conditions and in the presence of H 2 O 2 . High-performance liquid chromatography profiling of cell surface extracts identified unique peaks that were missing in the mutant strains, and partial characterization of the purified product from UA159 demonstrated that it is indeed a hybrid NRP/PK, as predicted. A genomic survey of 94 clinical S. mutans isolates suggests that the TnSmu2 gene cluster may be more prevalent than previously recognized.

Regulation of Bacteriocin Production and Cell Death by the VicRK Signaling System in Streptococcus mutans

“…In strain UA159, genes in TnSmu2 encode nonribosomal peptide synthetases (NRPS), polyketide synthases (PKS), and accessory proteins involved in nonribosomal peptide (NRP) and polyketide (PK) biosynthesis (56). These were shown to be necessary for the biosynthesis of a hybrid NRP/PK pigment that was referred to as mutanobactin (56), whereas the secondary metabolite generated from this cluster was isolated and characterized as mutanobactin A (17).…”

“…These were shown to be necessary for the biosynthesis of a hybrid NRP/PK pigment that was referred to as mutanobactin (56), whereas the secondary metabolite generated from this cluster was isolated and characterized as mutanobactin A (17). Whereas mutanobactin A was shown to suppress the morphological transition of Candida albicans from yeast to mycelium (17), mutational analysis of genes within the mutanobactin locus was shown to modulate biofilm formation and confer oxygen and hydrogen peroxide tolerance in S. mutans (56). In fact, we had previously reported the involvement of the VicRK in oxidative stress tolerance in S. mutans (48).…”

“…In strain UA159, genes in TnSmu2 encode nonribosomal peptide synthetases (NRPS), polyketide synthases (PKS), and accessory proteins involved in nonribosomal peptide (NRP) and polyketide (PK) biosynthesis (56). These were shown to be necessary for the biosynthesis of a hybrid NRP/PK pigment that was referred to as mutanobactin (56), whereas the secondary metabolite generated from this cluster was isolated and characterized as mutanobactin A (17). Whereas mutanobactin A was shown to suppress the morphological transition of Candida albicans from yeast to mycelium (17), mutational analysis of genes within the mutanobactin locus was shown to modulate biofilm formation and confer oxygen and hydrogen peroxide tolerance in S. mutans (56).…”

CovR Alleviates Transcriptional Silencing by a Nucleoid-Associated Histone-Like Protein in Streptococcus mutans

“…S. mutans contains two such genomic islands, TnSMu1 and TnSMu2; the exact function of these genomic islands is currently unknown. The locus studied here is a part of the TnSMu2 island and is responsible for production of the antimicrobial compound mutanobactin A (76). We speculate that TnSMu2 is also involved in iron acquisition, since a recent report suggests that a similar locus in Streptococcus equi is involved in both antimicrobial compound production and iron acquisition (40).…”

“…TnSmu2 possesses a gene cluster that spans from SMU.1334 to SMU.1348, and this gene cluster encodes enzymes related to nonribosomal peptide synthetase (NRPS) and polyketide synthase (PKS) and is involved in the synthesis of mutanobactin A, an NPRS-PKS metabolite (43). The gene cluster, which was originally named the smt operon (15), was subsequently designated the mub operon to reflect its role in mutanobactin A synthesis (76). As the regulation of horizontally acquired genes is very important from the physiological point of view of the host bacteria, we investigated the role of CovR in the regulation of genes of the smt (mub) locus.…”