The ability of cells to discriminate between homologous and foreign genes manifests itself differently from one species to the next. Escherichia coli is well known for its ability to express genes from various donor species, including Bacillus subtilis1,2. In contrast, B. subtilis strongly discriminates against the expression of E. coli genes (there have been no corroborated reports of the expression of a wholly intact E. coli gene in B. subtilis) but expresses genes from other Gram-positive bacteria such as Staphylococcus aureus3. This apparent asymmetry in the expression of heterologous genes between E. coli and B. subtilis may reflect fundamental differences in the mechanisms of gene expression in these two model systems. Here, we have overcome the inability of B. subtilis to express E. coli chloramphenicol resistance (Cmr) by supplanting the native regulatory element(s) of this Tn9-derived gene with B. subtilis DNA fragments, which represents the first step towards dissecting the nature of this asymmetric barrier to gene expression.
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