Horizontal transfer of pathogenicity islands of a virulence plasmid in Rhododoccus equi-Origin of the virulence-associated gene family-

马红球菌是一种兼性的细胞内革兰氏阳性球菌，可引起1-3个月大的小马驹的化脓性肺炎和溃疡性肠炎。这种细菌存在于土壤中，偶尔会在其他家畜中引起肺脓肿和淋巴结炎。毒力相关抗原和毒力质粒的发现使马立克氏杆菌的分类成为可能。马立克氏杆菌强毒株系及分子流行病学研究。根据VapA和VAPB阳性马立克氏杆菌代表性菌株的限制性内切酶酶切图谱，我们发现VapA阳性马立克氏杆菌至少有15种不同的质粒型，而VAPB阳性马立克氏杆菌至少有33种不同的质粒型。对ATCC33701(VapA阳性的代表)和A3(VAPB阳性的代表)的毒力质粒的核苷酸序列分析表明，存在一个致病岛：其特征是G C含量显著低于该质粒的其余部分，并且其两侧有类似于转座子解析酶的基因。本研究表明，毒力质粒核苷酸序列的测定和分析为了解马立克氏杆菌毒力质粒的多样性提供了信息。

Rhodococcus equi is a facultative, intracellular, Gram-positive coccobacillus that causes suppurative pneumonia and ulcerative enteritis in foals aged 1-3 months. This organism is present in soil and sporadically causes pulmonary abscesses and lymphadenitis in other domestic animals. The discovery of virulence-associated antigens and virulence plasmids has allowed the classification of R. equi. strains and molecular epidemiological studies of virulent R. equi. By the basis of restriction fragment patterns of representative strains of VapA-and VapB-positive R. equi digested by restriction enzymes, we found at least 15 distinct plasmid types in VapA-positive, and 33 types in VapB-positive R. equi. Nucleotide sequencing of the virulence plasmids of ATCC33701(the representative of VapA-positive) and A3(the representative of VapB-positive) revealed the presence of a pathogenicity island : it is characterised by a significantly lower G+C content than the rest of the plasmid, and is flankedby genes similar to transposon resolvases. The present study demonstraed that the determination and analysis of the nucleotide sequences of virulence plasmids provid the information on the diversity of virulence plasmids of R. equi.