A regulatory gene, reg1, was identified in Streptomyces lividans. It encodes a 345-amino-acid protein (Reg1) which contains a helix-turn-helix DNA-binding motif in the N-terminal region. Reg1 exhibits similarity with the LacI/GalR family members over the entire sequence. It displays 95% identity with MalR (the repressor of malE in S. coelicolor), 65% identity with ORF-Sl (a putative regulatory gene of ␣-amylase of S. limosus), and 31% identity with CcpA (the carbon catabolite repressor in Bacillus subtilis). In S. lividans, the chromosomal disruption of reg1 affected the expression of several genes. The production of ␣-amylases of S. lividans and that of the ␣-amylase of S. limosus in S. lividans were enhanced in the reg1 mutant strains and relieved of carbon catabolite repression. As a result, the transcription level of the ␣-amylase of S. limosus was noticeably increased in the reg1 mutant strain. Moreover, the induction of chitinase production in S. lividans was relieved of carbon catabolite repression by glucose in the reg1 mutant strain, while the induction by chitin was lost. Therefore, reg1 can be regarded as a pleiotropic regulatory gene in S. lividans.Streptomyces species, gram-positive soil bacteria with a high GC content, have the capacity to produce a vast array of secondary metabolites and extracellular proteins. The latter comprise many hydrolytic enzymes such as cellulases, chitinases, amylases, and xylanases allowing Streptomyces to grow on polymeric substrates (12). Genes encoding these enzymes are generally induced by degradation products of their polymeric substrates and repressed by readily metabolizable carbon sources such as glucose by a process known as catabolite repression. Several cis-acting sequences, displaying no apparent similarity, have been shown to be involved in the regulation of genes encoding such hydrolytic enzymes in various species of Streptomyces (10,11,44,49). However, no specific regulatory gene among those encoding extracellular proteins has so far been characterized for Streptomyces.␣-Amylases are secreted by several species of Streptomyces and catalyze the cleavage of the ␣-1,4 linkage of starch, yielding short linear maltodextrins which are finally converted to glucose. The highest ␣-amylase production was obtained when the ␣-amylase gene aml of Streptomyces limosus was cloned in S. lividans, which has a negligible ␣-amylase activity (43). In S. lividans, aml expression was induced by maltose and repressed by glucose. The regulation occurs at the level of transcription from the unique promoter. This led to the assumption that a regulatory protein(s) in S. lividans controlling the expression of aml might exist. A putative repressor, called open reading frame R (ORFR) in the present work, which is homologous to the regulatory proteins LacI and GalR of Escherichia coli has been located upstream of the ␣-amylase gene of S. venezuelae (45) and is also present upstream of the ␣-amylase genes of S. limosus (27) and S. griseus (42). Therefore, it was assumed that an ORF homo...