Studies on initiation of sporulation of B.cereus

蜡状芽孢杆菌在Ye-EMM中的IMP脱氢酶活性随Ye-EMM中细胞生长的增加而增加，而在Ye-EMM中蜡样芽孢杆菌不产生孢子。当蜡样芽孢杆菌在改良的G培养基上培养时，产孢量在6h时达到最高水平，之后逐渐下降。在1/100G培养基中营养下移诱导产孢量后，再悬浮1小时酶活力降至指数生长细胞的5%左右。蜡样芽孢杆菌在改良G和1/100G两种培养基上产孢率均在90%以上。霉酚酸在1 mM时对产孢量有强烈的抑制作用，而在1/100G中加入0和1小时的再悬浮液则抑制产孢量。蜡样芽孢杆菌胞内GTP浓度在再悬浮后1小时左右降至最低水平。在对照细胞中，虽然在再悬浮2小时时GTP增加到指数生长细胞的50%左右，但不增加1 mM霉酚酸的浓度。IMP脱氢酶是从蜡样芽孢杆菌细胞的粗提物中纯化出来的。用SDS-PAGE和凝胶过滤的方法测得该酶的相对分子质量分别为56 kDa和225 kDa。用枯草杆菌guaB探针筛选基因组文库，获得GR-1克隆。克隆的插入片段正在测序中。

IMP dehydrogenase activity of B.cereus increased with increase in cell growth in YE-EMM, where B.cereus did not sporulate. When B.cereus was cultured in a modified G medium, a sporulation medium, the activity reached the highest level at 6 hr and decreased thereafter. After induction of sporulation by nutritional shift down in 1/100 G medium, the enzyme activity decreased to about 5% of that of the exponentially growing cells at 1 hr of resuspension. The sporulation rate of B.cereus was over 90% in the modi- fied G medium and 1/100 G medium. The sporulation was strongly inhibited by mycophe- nolic acid at I mM when the drug was added at 0 and I hr of resuspension in 1/100 G medium. Intracellular GTP concentration of B.cereus decreased to the 1owest level about i hr of resuspension. Although the GTP increased to about 50% of the exponentially growing cells at 2 hr of resuspension in control cells, the concentration did not increase in the pres- ence of I mM mycophenolic acid.IMP dehydrogenase was purified from a crude extract of B.cereus cells. The molecular mass of the purified enzyme was estimated to be 56 kDa by SDS-PAGE and 225 kDa by gel filtration.B.cereus ts-4 genomic library was constructed with EMBL3 phage vector. By screening the genomic library with B.subtilis guaB probe, clone GR-1 was obatained. The insert of the clone is under sequencing.