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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.

Recessive mutations in the slyD gene were isolated by selecting for survival after induction of the cloned lysis gene E of bacteriophage phi X174 (Maratea, D., Young, K., and Young, R. (1985) Gene (Amst.) 40, 39-46). The slyD1 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 rather than for expression from the plasmid vector. The cloning, sequencing, and deletion analysis of a 1-kilobase pair genomic fragment containing the slyD locus, mapping at 73.5', is reported. Three reading frames, orf72, orf159, and orf196, are contained within this fragment, with the latter two reading frames occupying the same DNA on opposite strands. Deletion analysis shows that the complementing activity is restricted to the orf159/orf196 DNA. Complementation of the SlyD phenotype was observed irrespective of the orientation of the orf159/orf196 DNA with respect to a vector promoter, indicating that a cryptic promoter serves slyD on this fragment. Using site-directed mutagenesis, nonsense mutations were created in each reading frame which were silent in the opposing frame. Both orf196 nonsense alleles failed to complement slyD1, whereas both orf159 nonsense alleles retained complementation, demonstrating rigorously that orf196 is slyD. A segment corresponding to the first 150 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 COOH-terminal 46 codons of slyD encode a remarkable histidine-rich peptide sequence which is at least partly dispensable for slyD function in E-mediated lysis. Overexpression of slyD in E. coli is toxic. These findings are discussed in terms of a model for SlyD involvement in E function and in terms of a model for SlyD involvement of the ubiquitous FKBP rotamases.