Temperate Infection

In the lambda phage the DNA on entering the host cell (E.coli) may sometimes immediately multiply and enter the lytic cycle (virulent infection) described earlier. At other times (lysogenic cycle), however, the viral chromosome becomes a part of the host chromosome and is called the prophage. When the viral DNA is incorporated into the host DNA it behaves like a gene on the genetic map of the host. It replicates along with the host DNA. Thus the virus exists in two forms, the virion and the provirus (prophage in the case of bacterial viruses). The virion is the normal condition with a DNA core and a protein coat. The provirus is the intracellular form in which the viral DNA is a gene on the DNA of the host. Bacteria containing prophages are called lysogenic bacteria. Viruses whose chromosomes become prophages are called lysogenic viruses.



The lambda phage DNA is inserted into the E.coli DNA at a specific site between the gal operon and the bio operon. The insertion of phage DNA requires a specific enzyme, which has been called integrase. Removal of phage DNA from the bacterial chromosome requires intergrase and a second viral gene enzyme called excisions.



Excision is almost always precise. The phage DNA breaks away from the bacterial DNA exactly at the point of entry. In rare abnormal excisions the break occur in such a way that the circular phage DNA molecule has a part of bacterial DNA, and vice versa. ‘Transducing’ virus variants containing phage DNA along with a part of the gal gene have been found. When lambda DNA is mixed with lambda-gal DNA (hybridization), the two DNAs under complementary pairing, except in the gal region. In such heteroduplexes the two DNAs remains as separate single strands in the gal region, forming a loop called the substitution bubble.