Assembly of bacteriophage lambda heads: Protein processing and its genetic control in petit λ assembly

Abstract

Petit bacteriophage λ is a hollow λ head precursor which is found in λ-infected lysates, including lysates of phage λ carrying mutations in head genes. Wild-type petit λ has a protein composition similar to heads, except that it is missing pD § § Abbreviations used: pA, pB, etc., the protein products of genes A, B, etc; A − petit λ, B − petit λ, etc., petit λ purified from lysates of lysogenic cells in which the induced prophage carried an amber mutation in the indicated gene; groE − petit λ are obtained by induction of a groE mutant of E. coli lysogenic for wild-type λ. , a major component of heads. About 95% of the mass of petit λ is pE, the major structural protein of heads, and in addition it has proteins pB, h3, X1, and X2. Tryptic fingerprint analysis shows that h3 is a proteolytic cleavage product of pB, and previous experiments have shown that X1 and X2 are protein fusion products, closely related to each other and containing amino acid sequences of both pC and pE. Petit lambdas derived from infection by phages defective in genes A or D are indistinguishable from wild-type petit λ. B −, C −, or groE − defective petit lambdas show differences from wild-type in protein composition and in extent of protein processing. On the basis of the properties of mutant petit lambdas it is concluded that: (1) the protein processing reactions (cleavage of pB; fusion of pC with pE) occur on the petit λ structure; (2) cleavage of pB requires the functioning of genes C and groE but not A or D; (3) fusion of pC and pE requires gene groE but not A, B or D; (4) pNu3 participates directly in petit λ assembly but is lost from the structure by the time assembly is complete. Physical studies of petit λ show that wild-type, A −, B − and D − petit lambdas sediment at 150 S, while C − and groE − petit lambdas sediment at 190 S. Purified petit λ of either class has an ultraviolet absorption spectrum characteristic of pure protein.