影响目的基因在大肠杆菌中表达的因素.
参考答案：
1.外源基因的拷贝数2.外源基因的表达效率3.表达产物的稳定性4.细胞的代谢负荷
下列有关C4和C3植物的描述，错误的是[ ]
A．C3植物维管束鞘细胞中含有叶绿体
B．C4植物维管束鞘细胞中含有叶绿体
C．C3植物叶肉细胞中含有叶绿体
D．C4植物叶肉细胞中含有叶绿体
The workers ___ busily when the boss came to look for something she ___ in the office.
A．had worked; had left
B．were working; had left
C．working; had left
D．had worked; left
