Affiliation:
1. Department of Microbiology and Immunology, University of Illinois, Chicago 60612-7340, IL, USA
Abstract
Bacterial resistance to Ag(I) has been reported periodically with isolates from many environments
where toxic levels of silver might be expected to occur, but initial reports were limited to the
occurrence of resistant bacteria. The availability of silver-resistance conferring DNA sequences now
allow genetic and mechanistic studies that had basically been missing. The genes determining Ag(I)
resistance were sequenced from a plasmid found in a burn ward isolate. The 14.2 kb determinant
contains seven recognized genes, arranged in three mRNA transcriptional units. The silE gene
determines an extracellular (periplasmic space) metal-binding protein of 123 amino acids, including
ten histidine residues implicated in Ag(I) binding. SilE is homologous to PcoE, of copper resistance.
The next two genes, silR and silS, determine a two protein, histidine-kinase membrane sensor and aspartyl phosphate transcriptional responder, similar to other two component systems such as CzcR and CzcS (for cadmium, zinc and cobalt resistance) and PcoR and PcoS (for copper resistance).
The remaining four genes, silCBAP, are co-transcribed and appear to determine Ag+
efflux, with
SilCBA homologous to CzcCBA, a three component cation/proton antiporter, and SilP a novel P-type
ATPase with a amino-terminal histidine-rich cation-specificity region. The effects of increasing
Ag+
concentrations and growth medium halides (Cl-, Br- and I-)
have been characterized, with lower
Cl-
concentrations facilitating resistance and higher concentrations toxicity. The properties of this
unique Ag(I)-binding SilE protein are being characterized. Sequences similar to the silver-resistance
DNA are being characterized by Southern blot DNA/DNA hybridization, PCR in vitro DNA
synthesis and DNA sequencing. More than 25 additional closely related sequences have been
identified in bacteria from diverse sources. Initial DNA sequencing results shows approximately 5-20% differences in DNA sequences.
Subject
Inorganic Chemistry,Drug Discovery,Pharmacology,Toxicology
Cited by
43 articles.
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