Question

Phage gene expression is sometimes controlled by sigma factor switching and in other cases by antitermination. Describe these two regulatory processes and explain how they differ from each other.

Note

Please give detail and comprehensive explanation to the question.

Answer 1

The phage particle consists of a head [also known as capsid], a tail and tail fibers. The head contains the phage's double-strand linear DNA genome. During infection, the phage particle recognizes and binds to the host bacteria. The phage DNA is ejected through the tail into the bacterial cytoplasm, where the lambda DNA is replicated and new phage particles are produced which are ultimately released by the lysis of the bacterial cell.

Sometimes, under certain conditions, the viral DNA integrates itself into the host cell chromosome and is now called a 'prophage'. In this condition, it lies dormant without harming the host. The host is termed a 'lysogen' when a prophage is present within it. When the lysogen enters a stressed condition, the prophage might become active and enter the lytic cycle.

Sigma factors are multi-domain subunits of bacterial RNA polymerase[RNAP] that play critical roles in transcription initiation, including the recognition ad opening of promoters as well as the initial steps in RNA synthesis. Sigma factors are controlled by their sequestration by anti-sigma factors that occlude their RNAP-binding determinants.

Antitermination is a mechanism to fix premature termination of RNA synthesis during the transcription of RNA.

It occurs when the RNA polymerase ignores the termination signal and continues to elongates its transcript. This stops only when a second signal is received.

Antitermination provides a mechanism whereby one or more genes at the end of an operon can be switched either on or off, depending on whether the polymerase ither recognizing or not recognizing the termination signal.

A very few of the phage genes can be transcribed by the bacterial host RNA polymerase. Among these genes are regulators whose products allow the next set of phage genes to be expressed. Antitermination protein is one such regulator. In the absence of antitermination protein, RNA polymerase terminates at the terminator and when the antitermination protein is present, it continues past the terminator. Moreover, antitermination protein produced at each stage is specific for the particular transcription units that are expressed at that stage.

On the other hand, sigma factor is a protein needed only for initiation of transcription that enables specific binding of RNA polymerase to gene promoters.

So, the two mechanisms work for the same process but by different ways.