Affiliation:
1. School of Molecular and Cellular Biosciences, University of Natal, Pietermaritzburg, Scottsville 3209, South Africa
Abstract
ABSTRACT
Streptococcus milleri
NMSCC 061 produces an endopeptidase, millericin B, which hydrolyzes the peptide moiety of susceptible cell wall peptidoglycan. The nucleotide sequence of a 4.9-kb chromosomal region showed three open reading frames (ORFs) and a putative tRNA
Leu
sequence. The three ORFs encode a millericin B preprotein (MilB), a putative immunity protein (MilF), and a putative transporter protein (MilT). The
milB
gene encodes a 277-amino-acid preprotein with an 18-amino-acid signal peptide with a consensus IIGG cleavage motif. The predicted protein encoded by
milT
is homologous to ABC (ATP-binding cassette) transporters of several bacteriocin systems and to proteins implicated in the signal-sequence-independent export of
Escherichia coli
hemolysin A. These similarities strongly suggest that the
milT
gene product is involved in the translocation of millericin B. The gene
milF
encodes a protein of 302 amino acids that shows similarities to the FemA and FemB proteins of
Staphylococcus aureus
, which are involved in the addition of glycine to a pentapeptide peptidoglycan precursor. Comparisons of the cell wall mucopeptide of
S. milleri
NMSCC 061(resistant to lysis by millericin B) and
S. milleri
NMSCC 051(sensitive) showed a single amino acid difference. Serial growth of
S. milleri
NMSCC 051 in a cell wall minimal medium containing an increased concentration of leucine resulted in the in vivo substitution of leucine for threonine in the mucopeptide of the cell wall. A cell wall variant of
S. milleri
NMSCC 051 (sensitive) that contained an amino acid substitution (leucine for threonine) within its peptidoglycan cross bridge showed partial susceptibility to millericin B. The putative tRNA
Leu
sequence located upstream of
milB
may be a cell wall-specific tRNA and could together with the
milF
protein, play a potential role in the addition of leucine to the pentapeptide peptidoglycan precursor and thereby, contributing to self-protection to millericin B in the producer strain.
Publisher
American Society for Microbiology
Subject
Ecology,Applied Microbiology and Biotechnology,Food Science,Biotechnology
Cited by
24 articles.
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