Transcriptional Control in Bacteriophage Lambda

Bacteriophage X is a virus that infects the bacterium E. coli (Chapter 8). Bacteriophage X possesses a single DNA chromosome consisting of 48,502 nucleotides surrounded by a protein coat. A bacteriophage infects a bacterial cell by attaching to the cell wall and injecting its DNA into the cell. Inside the cell, X phage undergoes either of two life cycles.

In the lytic cycle (see Chapter 8), phage genes are transcribed and translated to produce phage coat proteins and enzymes that synthesize from 100 to 200 copies of the phage DNA. The viral components are assembled to produce phage particles, and the phage produces a protein that causes the cell to lyse. The released phage can then infect other bacterial cells. In the lysogenic cycle, phage genes that encode replication enzymes and phage proteins are not immediately transcribed. Instead, the phage DNA integrates into the bacterial chromosome as a prophage. When the bacterial chromosome replicates, the prophage is duplicated along with the bacterial genes and is passed to the daughter cells in bacterial reproduction. The prophage may later excise from the bacterial chromosome and enter the lytic cycle.

Whether a X phage enters the lytic or the lysogenic cycle depends on the regulation of the phage genes. In the lytic cycle, the genes that encode replication enzymes, phage proteins, and bacterial cell lysis are transcribed; but, in the lysogenic cycle, these genes are repressed.

Like bacterial genes, functionally related phage genes are clustered together into operons. There are four major operons in the phage X chromosome (< Figure 16.18). The early right operon contains genes that are required for DNA replication and are transcribed early in the lytic cycle. The early left operon contains genes necessary for recombination and the integration of phage DNA into the bacterial chromosome as a part of the lysogenic cycle. A third operon, the late operon, contains genes that encode the protein coat of the phage, produced late in the lytic cycle. The fourth operon is the repressor operon, which produces the X repressor responsible for maintaining the prophage DNA in a dormant state. Although there are several additional promoters on the X chromosome that may be activated at special times, here the emphasis is on three general features of transcriptional control in bacteriophage X.

First, both positive control and negative control are seen in X gene regulation. Several proteins act as repressors, inhibiting transcription, whereas others act as activators,

0 0

Post a comment