Novel Type V Staphylococcal Cassette Chromosome mec Driven by a Novel Cassette Chromosome Recombinase, ccrC

Abstract

Staphylococcal cassette chromosome mec (SCC mec ) is a mobile genetic element composed of the mec gene complex, which encodes methicillin resistance, and the ccr gene complex, which encodes the recombinases responsible for its mobility. The mec gene complex has been classified into four classes, and the ccr gene complex has been classified into three allotypes. Different combinations of mec gene complex classes and ccr gene complex types have so far defined four types of SCC mec elements. Now we introduce the fifth allotype of SCC mec , which was found on the chromosome of a community-acquired methicillin-resistant Staphylococcus aureus strain (strain WIS [WBG8318]) isolated in Australia. The element shared the same chromosomal integration site with the four extant types of SCC mec and the characteristic nucleotide sequences at the chromosome-SCC mec junction regions. The novel SCC mec carried mecA bracketed by IS 431 (IS 431 - mecA -Δ mecR1- IS 431 ), which is designated the class C2 mec gene complex; and instead of ccrA and ccrB genes, it carried a single copy of a gene homologue that encoded cassette chromosome recombinase. Since the open reading frame (ORF) was found to encode an enzyme which catalyzes the precise excision as well as site- and orientation-specific integration of the element, we designated the ORF cassette chromosome recombinase C ( ccrC ), and we designated the element type V SCC mec . Type V SCC mec is a small SCC mec element (28 kb) and does not carry any antibiotic resistance genes besides mecA . Unlike the extant SCC mec types, it carries a set of foreign genes encoding a restriction-modification system that might play a role in the stabilization of the element on the chromosome.