The ethanolamine utilization (eut) operon of Salmonella typhimurium is controlled by a positive regulatory protein (EutR) which stimulates eut operon expression in response to the simultaneous presence of two effectors, ethanolamine and adenosyl-cobalamin (Ado-B12). Ado-B12 is a cofactor for ethanolamine ammonialyase (lyase), the first enzyme in the ethanolamine-degradative pathway. The
d
-Fucose, a nonmetabolizable analogue of
l
-arabinose, prevents growth of
Escherichia coli
B/r on a mineral salts medium plus
l
-arabinose by inhibiting induction of the
l
-arabinose operon. Mutations giving rise to
d
-fucose resistance map in gene
araC
and result in constitutive expression of the
l
-arabinose operon. Most of these mutations also permit
d
-fucose to serve as a gratuitous inducer. It is concluded that
d
-fucose-resistant mutants produce an
araC
gene product with an altered inducer specificity. Addition of
l
-arabinose to cells induced with the gratuitous inducer,
d
-fucose, resulted in severe transient repression of operon expression followed by permanent catabolite repression. Transient repression but no permanent catabolite repression was obtained when cells unable to metabolize
l
-arabinose were employed. It is concluded that transport of
l
-arabinose alone is sufficient to achieve transient repression of its own operon, but that metabolism of
l
-arabinose must occur to achieve permanent catabolite repression of the
l
-arabinose operon. This general effect has been termed “self-catabolite repression.”
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