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Journal of Bacteriology, August 1999, p. 4452-4460, Vol. 181, No. 15
Institut für Mikrobiologie,
Forschungszentrum für Milch und Lebensmittel Weihenstephan,
Technische Universität München, D-85350 Freising, Germany
Received 1 March 1999/Accepted 28 April 1999
We have cloned, sequenced, and characterized the genes encoding the
lytic system of the unique Staphylococcus aureus phage 187. The endolysin gene ply187 encodes a large cell wall-lytic enzyme (71.6 kDa). The catalytic site, responsible for the hydrolysis of staphylococcal peptidoglycan, was mapped to the N-terminal domain of
the protein by the expression of defined ply187 domains. This enzymatically active N terminus showed convincing amino acid sequence homology to an
N-acetylmuramoyl-L-alanine amidase, whereas the
C-terminal part, whose function is unknown, revealed striking relatedness to major staphylococcal autolysins. An additional reading
frame was identified entirely embedded out of frame (+1) within the 5'
region of ply187 and was shown to encode a small, hydrophobic protein of holin-like function. The hol187 gene
features a dual-start motif, possibly enabling the synthesis of two
products of different lengths (57 and 55 amino acids, respectively).
Overproduction of Hol187 in Escherichia coli resulted in
growth retardation, leakiness of the cytoplasmic membrane, and loss of
de novo ATP synthesis. Compared to other holins identified to date,
Hol187 completely lacks the highly charged C terminus. The secondary structure of the polypeptide is predicted to consist of two small, antiparallel, hydrophobic, transmembrane helices. These are supposed to
be essential for integration into the membrane, since site-specific introduction of negatively charged amino acids into the first transmembrane domain (V7D G8D) completely abolished the function of the
Hol187 polypeptide. With antibodies raised against a synthetic 18-mer
peptide representing a central part of the protein, it was possible to
detect Hol187 in the cytoplasmic membrane of phage-infected S. aureus cells. An important indication that the protein actually functions as a holin in vivo was that the gene (but not the V7D G8D
mutation) was able to complement a phage
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Evidence for a Holin-Like Protein Gene Fully Embedded Out of
Frame in the Endolysin Gene of Staphylococcus aureus
Bacteriophage 187
Sam mutation
in a nonsuppressing E. coli HB101 background. Plaque
formation by
gt11::hol187 indicated that both
phage genes have analogous functions. The data presented here indicate
that a putative holin is encoded on a different reading frame within
the enzymatically active domain of ply187 and that the
holin is synthesized during the late stage of phage infection and found
in the cytoplasmic membrane, where it causes membrane lesions which are
thought to enable access of Ply187 to the peptidoglycan of
phage-infected Staphylococcus cells.
*
Corresponding author. Mailing address: Institut
für Mikrobiologie, Forschungszentrum für Milch und
Lebensmittel Weihenstephan, Technische Universität München,
Weihenstephaner Berg 3, D-85350 Freising, Germany. Phone:
49-8161-71-3859. Fax: 49-8161-71-4492. E-mail:
M.J.LOESSNER{at}LRZ.TUM.DE.
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