The phage growth limitation (Pgl) system, encoded by Streptomyces coelicolor A3(2), confers protection against the temperate bacteriophage phi C31 and its homoimmune relatives. The Pgl phenotype is characterized by the ability of Pgl(+) hosts to support a phage burst on initial infection but subsequent cycles are severely attenuated. Previously, two adjacent genes pglY and pglZ were shown to be required for Pgl. It had been shown by Southern blotting that Streptomyces lividans , a close relative of S. coelicolor and naturally Pgl(-) , does not contain homologues of pglYZ and that introduction of pglYZ into S. lividans is not sufficient to confer a Pgl(+) phenotype. Moreover, the mechanism of the Pgl(+) <-> Pgl(-) phase variation associated with this phenotype is also not understood. Here we describe two novel genes, pglW and pglX , that were shown to be part of this system by complementation of Pgl(-) mutants and by insertional mutagenesis. pglW encodes a 169 kDa protein that includes putative motifs for both serine/threonine protein kinase activity and DNA binding. pglX encodes a 136 kDa protein with putative adenine-specific DNA methyltransferase activity. pglW and pglX have overlapping stop-start codons suggesting transcriptional and translational coupling. S1 mapping of transcripts initiating up-stream of pglW indicated that, like pglYZ , pglWX is expressed in uninfected cultures. A homologue of pglX with 76% amino acid identity was identified in S. coelicolor , and insertional mutagenesis indicated that this gene was not required for the Pgl(+) phenotype. Southern blots indicated that S. lividans does not contain homologues of pglW or pglX . A plasmid encoding pglWXYZ was able to confer the Pgl(+) phenotype to S. lividans implying that these four genes constitute the whole system.
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