Transcription of the Zac Operon of Escherichia coZi*

The effects of induction and of transient and catabolite repression on lac-specific RNA in cells of Escherichia coli were investigated. The rate of Zac-specific RNA synthesis increases linearly for 3 min after induction and then becomes constant. The accumulation of RNA complementary to the promoter proximal tat 2 gene begins before accumulation of RNA complementary to the promoter distal lac Y-A genes. When transient repression is imposed upon the cells by the addition of glucose, the lac Z-specific RNA begins to decay before the lac Y-A-specific RNA. Catabolite repression reduces the rate of formation of lac-specific RNA relative to the rate of formation of bulk RNA. Both lac Z-specific RNA and lac Y-A-specific RNA are equally aBected. These results confirm the concept that the sole effect of transient repression and of catabolite repression is the inhibition of the initiation-of RNA synthesis at the lac promoter site.