A mutant which produces a small-headed ("petite") variant of bacteriophage T4 is described. The mutation (E920g) maps in a new gene (66) between genes 23 and 24. Petite phage particles composed up to 70% of the phage yield. The petite phage was nonviable upon single infection but produced progeny when two or more infected a cell. Its genome was shortened by a random deletion of about 30%, and deoxyribonucleic acid (DNA) extracted from the particles was 0.68 the length of normal T4 DNA. The

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... tion in DNA length was accompanied by a proportional reduction in head volume. Double mutants between E920g and head-defective mutants in gene 21 produced unusually high frequencies of spherical capsidlike structures (r-particles). I Permanent address: Department of Biophysics, University of Chicago, Chicago, Ill. 60637. MATERIALS AND METHODS Media. The composition of the EHA top and bottom agar used was given by Steinberg and Edgar (31); the composition of M9S was given by Champe and Benzer (3). Bacterial and phage strains. In most experiments, and for the preparation of stocks, the three Escherichia coli strains B, S/6, and CR63 were employed. Methods for the preparation of host cells and indicators were given by Epstein et al. (8). The strains E. coli Hfr H(X), sui-(CA244), and sui+ (CA267), an isogenic sult pair, were the kind gift of S. Brenner. The T4 mutants used are shown in Fig. 3 . Their characteristics and the plating conditions used were described previously (8). The T4 mutant E920g (Geneva) was derived from a mutant E920 (isolated by R. S. Edgar) in a series of backcrosses to wildtype phage. The final derivative was called E920/96/ 41, shortened to E920g. The original mutant E920 isolated by Edgar yielded a high frequency of short-headed "petite" phage (65 to 90% of the complete particles in a lysate) when grown on E. coli B. Analysis of the backcrosses showed that the original mutant E920 contained, in addition to the mutation causing petite phage production, two minute-plaque mutants and a temperature-sensitive (ts) mutant. Neither the ts mutant nor the one minute-plaque mutant which was tested produced petite phage. Stocks of mutant E920g for crosses and complementation tests were prepared with E. coil CR63 grown in M9S as host. The mutant E920g obtained from the backcrosses can be identified by its minute-plaque type on S/6, either by use of standard plating conditions or, more readily, after long incubation at 37 C on 4to 5-day-old plates 865 on May 9, 2020 by guest