嗜热脂肪芽孢杆菌CU21中有关质粒pFDX163的整合和游离状态的一种新的相转变现象

pFDX1是带有外源基因xyIE的重组质粒。从嗜热脂肪芽孢杆菌CU21(pFDX1)出发,提高培养温度后选择卡那霉素抗性突变菌落,得到一个变异株CU21-163。该菌株含有突变质粒pFDX163,它由一个来自宿主基因组的2.0kb的H片段插入pFDX1而构成。pFDX163可通过H片段的同源重组而整合到染色体上。CU21-163包含y、w两类细胞,二者的xyIE基因表达量有明显差异。这两类细胞在分裂过程中呈现一种新的相转变现象,即y细胞的后代中经常出现少数w细胞,w细胞的后代中经常出现少数y细胞。对CU21-163的不同细胞群体的总DNA样品中游离质粒与整合质粒的含量进行了测定,由此推断y细胞含有游离质粒和整合质粒,w细胞只含整合质粒。

A Novel Type of Phase Variation Regarding Integrated and Free States of Plasmid pFDX163 in Bacillus stearothermophilus CU21

pFDXl is a recombinant plasmid which carries a foreign gene xylE. By selecting for kana-mycin-resistant mutants of Bacillus stearothermophilus CU21(pFDXl) at higher temperature, a variant strain CU21-163 was obtained. This strain harbors a mutant plasmid pFDX163, which was formed by insertion of a 2.0kb H-fragment from the CU21 genome onto the plasmid pFDXl. pFDX163 was supposed to be integrated into the CU21 chromosome via homologous recombination of H-fragments. The CU21-163 strain consists of two cell types, i.e. y-cell and w-cell. The expression level of xylE gene in the former is higher than that in the latter. The progeny of a y-cell always contains some w-cells, while that of a w-cell contains y-cells. This is supposed to be due to a phase variation of CU21-163. Analysis on the amount of free and integrated plasmid DNA in different DNA samples of CU21-163 cells allows us to draw the conclusion that there are both free and integrated plasmids in the y-cells, whereas only integrated ones in the w-cells.