The origin and mechanism of transfer of a new mobile genetic element SCCmec

我们测定了从美国芝加哥儿童医院分离的三株MRSA株CA05和8/6-3P以及从日本门诊分离的81/108株SCCmecs的全核苷酸序列。3株SCCmecs均携带2型CcrA、B基因和由IS1272-ΔMecri-mecA-IS431组成的B类mec基因复合体。3个SCCmecs(除Ccr基因复合体和mec基因复合体外)的J区与已报道的3个SCCmecs(I型、II型和III型)不同。因此，我们将CA05的SCCmec命名为IVa型，将8/6-3P的SCCmec命名为IVb型，将81/108命名为IVc型。通过聚合酶链式反应实验，我们发现3株甲氧西林敏感型菌株，人链球菌、阿莱特链霉菌和精链球菌都携带Ccr基因。我们选择了一株人链球菌GIFU12263，测定了该菌株Ccr基因周围的核苷酸序列，并鉴定了一个独特的结构，命名为scc_&lt；12263&gt；我们认为SCC_&lt；12263&gt；是葡萄球菌盒带染色体(SCCmec)活跃的原始可移动遗传元件。我们从人链球菌GIFU12263中克隆了ccrA1和ccrB1基因，从CA05和81/108中克隆了ccrA2和ccrB2基因。这些ORF可以催化从耐甲氧西林的I型葡萄球菌和IV型葡萄球菌中精确切割SCCmec，并可将SCCmec从MRSA转导到MSSA。然而，SCCmec的接合转移尚未得到证实。

We have determined entire nucleotide sequences of SCCmecs in three MRSA strains, CA05 and 8/6-3P which were isolated from Chicago Children's Hospital, U.S.A, and 81/108 which was isolated form an outpatient in Japan. The SCCmecs of all three strains carried type-2 ccr A, B genes, and class B mec gene complex composed of IS1272-ΔmecRI-mecA-IS431. The J-regions of three SCCmecs(the region except ccr gene complex and mec gene complex in SCCmec) were not identical and differed from previously reported three SCCmecs(type-I, II, and III). Therefore, we have designated the SCCmec of CA05 as typeIVa, that of 8/6-3P as typeIVb, and 81/108 as typeIVc respectively. By PCR experiment, we found that three methicilin-susceptible type strains, S. hominis, S. arlettae, and S. arincularis, carried ccr genes. We selected a strain, S. hominis GIFU12263, determined the nucleotide sequence around ccr gene of the strain, and identified an unique structure designated SCC_<12263>. We consider that SCC_<12263> is an active primordial mobile genetic element for the Staphylococcal Cassette Chromosome mec (SCCmec). We have cloned ccrA1 and ccrB1 genes from S. hominis GIFU12263 as well as ccrA2 and ccrB2 genes from CA05 and 81/108. Those orfs were found to catalyze precise excision of the SCCmec from the methicillin-resistant staphylococci Type-I and Type-IV SCCmec could be transduced from MRSA into MSSA. However, conjugative transfer of SCCmec was not proved yet.