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Recessive mutations in the slyD (sensitivity to lysis) gene were isolated by selecting for survival after induction of the cloned lysis gene E of bacteriophage phi X174 [1]. The slyD- mutation, transduced into the normal phi X174 host, Escherichia coli C, confers an absolute block on the plaque-forming ability of the wild-type phage, indicating that slyD is required for E function. slyD encodes a protein with 196 residues. A segment corresponding to the first 142 residues of the predicted SlyD protein has significant similarity throughout its length to the FKBP family of peptidyl-prolyl cis-trans isomerases, or rotamases. The C-terminal 46 codons of slyD encode a remarkable histidine-rich peptide which is a metal-binding domain [2]. This sequence is dispensable for slyD function in E-mediated lysis. Although there is no obvious phenotype associated with the slyD- genotype other than the resistance to E-mediated lysis, overexpression of slyD causes cells to filament and to increase significantly in diameter. Mutations in phi X174 can restore the plaque-forming ability of the phage on a slyD- host. These pos (plates on slyD) mutants plate on E. coli C wild-type and slyD-. A model for SlyD involvement in E function and the role of SlyD in the cell is discussed.