A mutation, designated hutCR11, which resulted in high expression of the hut operon and release of the catabolite repression and amino-acid repression of hut expression, was isolated and determined to be a T-to-G transversion at position +30 (+1 indicates the transcription-initiation site). In the hutCR11 mutant, levels of hutP mRNA were 5-fold higher than those in wild-type cells under conditions of non-induction and induction and Ii-fold higher under conditions of catabolite repression and amino-acid repression. Mutation analysis showed that two types of base change (T --> A and T --> C) at position +30 did not cause high expression of the hut operon, indicating that this was specifically caused by the single base substitution (T --> G) at position +30. The base substitution of A for T at position +30 also led to partial relief of both catabolite repression and amino-acid repression. These results indicate that the nucleotide sequence at +30 is important for regulation of both catabolite repression and amino-acid repression of the hut operon.