Identification of Proteins Coded in Phage Lambda

Abstract

Phage λ has sufficient DNA to encode about 50 proteins of molecular weight 33,000, and about 35 genes are known. However, fewer than ten λ proteins have been studied in any detail, largely because of the lack of specific assays for the others, and because host protein synthesis continues after infection by λ. This paper describes the labeling, identification, and characterization of λ proteins by a technique devised by Ptashne which circumvents these problems. The proteins identified belong to both the early and late classes, and several are products of genes which have not been described previously.

EXPERIMENTAL METHOD
The basic method used here to characterize λ proteins is to label them with radioactive amino acids and to compare the electrophoretic patterns of radioactive proteins obtained from various mutants. These experiments depend on the fact, first reported by Ptashne, that infection by wild-type λ (λ⁺), of cells heavily irradiated with ultraviolet light (UV), stimulates the ability of the cells to incorporate radioactive leucine into TCA-insoluble material by 10–15-fold (Ptashne, 1967). This technique makes it possible to label phage-specific proteins without an intolerable level of labeling of Escherichia coli proteins.

Synthesis of Early and Late λ Proteins in Irradiated Bacteria
Figure 1 shows the dependence of total leucine incorporation on UV dose for uninfected cells and for cells infected with three different phages. The ratio of incorporation between λ⁺-infected and uninfected cells can be seen to increase with dose up to about 12,000 erg/mm² and then to remain approximately constant.