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Concepts EH

Antisense RNA is complementary to other RNA or DNA sequences. In bacterial cells, it may inhibit translation by binding to sequences in the 5' UTR of mRNA and preventing the attachment of the ribosome.

Regulator of early left genes

Repressor operon

Genes for integrating viral DNA into bacterial chromosome

Repressor operon

Regulator of %

Phage DNA replication proteins

Regulator of early left genes

Genes for viral head proteins

Genes for viral tail proteins

Regulator of %

Phage DNA replication proteins

Regulator of late genes

Genes for lysis proteins

Genes for viral head proteins

Genes for viral tail proteins

4 16.18 The bacteriophage X chromosome contains four major operons: the early left operon, the early right operon, the late operon, and the repressor operon.

stimulating transcription. The X repressor, which plays a major role in X gene regulation, can act as either an activator or a repressor.

A second feature is that transcription is accomplished through a cascade of reactions. As one operon is transcribed, it produces a protein that regulates the transcription of a second operon, which produces a protein that affects the transcription of a third operon. Thus, the operons are activated and repressed in a particular order, with the use of several different promoters, each with an affinity for specific activators and repressors. As each promoter is activated, only the genes under its control are transcribed;

this controlled transcription ensures that genes appropriate to each stage of the lytic or lysogenic cycle are expressed.

A third feature of X gene regulation is the use of transcriptional antiterminator proteins, which bind to RNA polymerase and alter its structure, allowing it to ignore certain terminators (<Figure 16.19a). In the absence of the antiterminator protein, RNA polymerase stops at a terminator located early in the operon (<Figure 16.19b), and so only some of the genes in the operon are transcribed and translated.

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