Prophage induction in strains 168, S31 and W23 is accompanied by synthesis of two endolysins. The synthesis of those of strain 168, with molecular masses of 32 and 34 kDa, was shown to be controlled by the repressor of the defective phage PBSX. The 32 kDa protein corresponds to an -acetylmuramoyl-L-alanine amidase, and plays the major role in PBSX-mediated lysis. Its structural gene, , is the last in the PBSX late operon, whose four most distal open reading frames have been cloned and sequenced. Analysis of the nucleotide sequence suggests that the two open reading frames preceding , designated and , encode polypeptides whose combined action could play the role of a holin. The open reading frame upstream of , designated , encodes an exoprotein. The phage amidase, although endowed with a signal peptide, is apparently, like Xep, exported by a holin-like mechanism which does not involve the cleavage of the signal peptide. The presence on the chromosome of other, similar, genes, and their possible widespread occurrence, is discussed.


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