Formation of the Tail
The tail of lambda is formed independently of the
head and is then attached to the head. The most striking property of tail
maturation is a mechanism that permits the tail to grow to a fixed length.
First, proteins pG, pH, pI, pJ, pK, pL, and pM interact to form an initiator of
polym-erization of the major tail protein, pV. The pV protein polymerizes on
this initiator to form the tail tube. Finally, the terminator of polymeri-zation,
pU, is added. After this, pZ protein functions, and the tail is attached to the
head. The tail fibers formed from the pTfa and pStf are added somewhere in this
sequence. Unbeknownst until recently, the lambda phage chosen for laboratory
work in the 70s and 80s lacked these fibers and adventitiously adsorbed, albeit
slowly, via the pJ fiber to the maltose porin in the outer cell wall. As a
result of this slow adsorption, the tail fiber-deficient lambda form large
plaques. Perhaps for the good of science, the resulting large-plaque morphology
made it possible to distinguish clear and turbid plaques, and hence, permitted
the study of lysogeny and its regulation.
If mutations in the tail genes are present,
abnormal tails may be formed. For example, if the terminator of polymerization,
pU, is absent, then normal tails are formed, but the pZ protein cannot function
and the phage is largely inviable. Upon prolonged incubation of U-
extracts, in vivo or in an in vitro reconstitution system, the
tails will extendbeyond their normal length to form what is called a polytail.
This structure is sufficiently normal that the pZ protein can function on it,
and the polytail is attached to the head to yield a phage particle, but one of
very low infectivity. Of course, if the pV protein is absent, no tail
polymerization occurs. Similarly, mutations in any
of the genes G through M block initiator formation and
therefore no tail is formed. The pH protein of the tail undergoes proteolytic
cleavage during maturation to yield a protein called H*.
Apparently the pH protein acts much like a tape measure in determining the
length of the tail. Internal deletions in H
yield phage with shorter tails.