Early Transcription of Genes N and Cro
Within a minute after injection into the host, the complementary ends of the lambda DNA anneal to each other and are covalently joined by the DNA ligase of the host. Supercoiling of the circle by DNA gyrase is then possible, and the DNA rapidly acquires the superhelical density necessary for maximum activity of the phage early promoters pL and pR. These are located on either side of CI.
As on all promoters, initiation of transcription from the promoters pLand pRis followed by the dissociation of the sigma subunit of RNApolymerase after the polynucleotide chains have reached a length of 6 to 15 nucleotides. Then a host protein, the NusA product, binds to RNA polymerase. Nus stands for N utilization substance. Most likely it binds in the site previously occupied by the sigma subunit because only one or the other of these proteins can bind to RNA polymerase at once.
The RNA polymerase molecules elongate past the point where the sigma and NusA subunits exchange and they transcribe through the N
gene on the left and the cro gene on the right. This is all that happens at this stage of the lambda infection, for just beyond these genes the RNA
Figure 14.4 The sigma subunit dissociates from RNA polymerase and is re-placed by the NusA protein. At the terminator, the rho protein interacts with the complex and releases RNA and RNA polymerase.
polymerase pauses, assisted by the NusA protein, and the host transcrip-tion terminator, rho, releases the RNA chain and RNA polymerase from the DNA (Fig. 14.4). This pausing and termination occurs at the first transcription termination sites in the pL and pR operons, tL1 and tR1. Additional terminators with slightly different properties, tL2 and tR2, lie farther downstream in both operons. As a result of tL1 and tR1, the first stage of lambda vegetative growth is confined to the accumulation of N and Cro proteins.
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