Expression of a Cryptic Secondary Sigma Factor Gene Unveils Natural Competence for DNA Transformation in Staphylococcus aureus

Abstract

It has long been a question whether Staphylococcus aureus, a major human pathogen, is able to develop natural competence for transformation by DNA. We previously showed that a novel staphylococcal secondary sigma factor, SigH, was a likely key component for competence development, but the corresponding gene appeared to be cryptic as its expression could not be detected during growth under standard laboratory conditions. Here, we have uncovered two distinct mechanisms allowing activation of SigH production in a minor fraction of the bacterial cell population. The first is a chromosomal gene duplication rearrangement occurring spontaneously at a low frequency [≤10−5], generating expression of a new chimeric sigH gene. The second involves post-transcriptional regulation through an upstream inverted repeat sequence, effectively suppressing expression of the sigH gene. Importantly, we have demonstrated for the first time that S. aureus cells producing active SigH become competent for transformation by plasmid or chromosomal DNA, which requires the expression of SigH-controlled competence genes. Additionally, using DNA from the N315 MRSA strain, we successfully transferred the full length SCCmecII element through natural transformation to a methicillin-sensitive strain, conferring methicillin resistance to the resulting S. aureus transformants. Taken together, we propose a unique model for staphylococcal competence regulation by SigH that could help explain the acquisition of antibiotic resistance genes through horizontal gene transfer in this important pathogen.

Plasmid-Encoded ComI Inhibits Competence in the Ancestral 3610 Strain of Bacillus subtilis

“…Why pBS32 encodes a functional inhibitor of competence is unknown, but ComI may promote plasmid stability by excluding the uptake of competing plasmids. Many bacteria, including pathogens like Staphylococcus aureus, Streptococcus pyogenes, and Listeria monocytogenes, are nontransformable and yet carry genes which suggest that they could be capable of natural competence (50)(51)(52). Similarly, industrial strains such as Bacillus amyloliquefaciens, Bacillus licheniformis, Bacillus pumilus, and Bacillus subtilis subsp.…”

Listeria monocytogenes σHContributes to Expression of Competence Genes and Intracellular Growth

“…Potential translation initiation codons (TTG) of sigH in S. aureus RN4220 were changed to Ochre stop codons (TAA) by using pMADt492 as described previously (21).…”

“…Among Gram-positive bacteria, two master regulators control the expression of competence-related genes: the transcription factor ComK (in B. subtilis) (18) and the alternative sigma factor H (in S. pneumoniae [19,20], S. aureus [21], and Lactobacillus sakei [31]). Rabinovich et al reported the presence of a ComK recognition site (the ComK box) upstream of the competence machinery operons (comG, comE, and comF) in L. monocytogenes (16).…”

“…In other Gram-positive species, the percentage of the population that expresses these genes and develops competence varies greatly. In B. subtilis and S. aureus, only a fraction of the cells can express competence genes (10 to 20% and 2%, respectively) (21,33). In contrast, in S. pneumoniae, all the cells can become competent (34).…”

“…In Bacillus subtilis, the main regulator is the transcription factor ComK (18), whereas Streptococcus pneumoniae (19,20) and Staphylococcus aureus (21) use the sigma factor ComX/ H as the key regulator. In addition, staphylococcal ComK participates in the expression of competence genes (22).…”