Abstract
To study the effect of gene dosage on gene expression, λplac5cI857O29P3, a replication defective lambda phage carrying part of the lac operon (containing the lac promoter, operator and z gene) in the b2 region was studied in Escherichia coli strain JC6256 where the lac operon is deleted and at a temperature where the λ repressor is inactive. In measuring the synthesis of -galactosidase, it was possible to separate the effects of the lac promoter from those of the phage promoter. When the synthesis of -galactosidase was initiated from the inserted lac promoter in JC6256(λ+) in the presence of additional cyclic AMP, the rate and level of -galactosidase synthesis were directly proportional to the multiplicity of infection (gene dosage). Furthermore, -galactosidase synthesis was initiated about 5 min after infection, just as with isopropyl--D-thiogalactoside (IPTG) induction.
When the synthesis of -galactosidase was initiated from the phage promoter in JC6256 in the absence of additional cyclic AMP, the rate and level of -galactosidase synthesis were again linearly proportional to gene dosage. On the other hand, initiation of -galactosidase synthesis was delayed until 10 to 20 min after infection. These results suggest that: (i) in the absence of negative controlling factors, the extent of gene expression is proportional to gene dosage; (ii) varying the gene dosage can be used to regulate gene expression.