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Journal of Bacteriology, April 2001, p. 2543-2552, Vol. 183, No. 8
0021-9193/01/$04.00+0 DOI: 10.1128/JB.183.8.2543-2552.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Characterization of the sat Operon
in Streptococcus mutans: Evidence for a Role of Ffh in
Acid Tolerance
Bas H. A.
Kremer,1,
Marieke
van
der Kraan,1
Paula J.
Crowley,1
Ian R.
Hamilton,2
L. Jeannine
Brady,1 and
Arnold S.
Bleiweis1,*
Department of Oral Biology, University of
Florida, Gainesville, Florida 32610,1 and
Department of Oral Biology, University of Manitoba, Winnipeg,
Canada2
Received 22 November 2000/Accepted 26 January 2001
An essential protein translocation pathway in Escherichia
coli and Bacillus subtilis involves the signal
recognition particle (SRP), of which the 54-kDa homolog (Ffh) is an
essential component. In a previous study, we found that a transposon
insertion in the ylxM-ffh intergenic region of the
designated secretion and acid tolerance (sat) operon of
Streptococcus mutans resulted in an acid-sensitive
phenotype. In the present study, we further characterized this genomic
region in S. mutans after construction of bonafide sat operon mutants and confirmed the role of the SRP
pathway in acid resistance. Northern blot and primer extension analyses
identified an acid-inducible promoter upstream of ylxM
that was responsible for upregulating the coordinate expression of all
five genes of the sat operon when cells were grown at
acid pH. Two constitutive promoters, one immediately upstream of
satD and one just 3' to the acid-inducible promoter,
were also identified. Except for Ffh, the functions of the
sat operon gene products are unknown. SatC, SatD, and
SatE have no homology to proteins with known functions, although YlxM
may function as a transcriptional regulator linked to genes encoding
SRP pathway proteins. Nonpolar mutations created in each of the five
genes of the sat locus resulted in viable mutants. Most
striking, however, was the finding that a mutation in
ffh did not result in loss of cell viability, as is the
case in all other microbial species in which this pathway has been described. This mutant also lacked immunologically detectable Ffh and
was severely affected in resistance to acid. Complementation of the
mutation resulted in restoration of acid tolerance and reappearance of
cytoplasmic Ffh. These data provide evidence that the SRP pathway plays
an important role in acid tolerance in S. mutans.
*
Corresponding author. Mailing address: Department of
Oral Biology, University of Florida, Gainesville, FL 32610. Phone:
(352) 846-0784. Fax: (352) 392-7357. E-mail:
bleiweis{at}dental.ufl.edu.

Present address: Division of Immunological and Infectious Diseases,
TNO-PG, 2301 CE Leiden, The
Netherlands.
Journal of Bacteriology, April 2001, p. 2543-2552, Vol. 183, No. 8
0021-9193/01/$04.00+0 DOI: 10.1128/JB.183.8.2543-2552.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
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