The nucleotide sequence of the coding and regulatory regions of the ca-amylase gene (aml) of Streptomyces limosus was determined. High-resolution Si mapping was used to locate the 5' end of the transcript and demonstrated that the gene is transcribed from a unique promoter. The predicted amino acid sequence has considerable identity to mammalian and invertebrate ca-amylases, but not to those of plant, fungal, or eubacterial origin. Consistent with this is the susceptibility of the enzyme to an inhibitor of mammalian a-amylases. The amino-terminal sequence of the extracellular enzyme was determined, revealing the presence of a typical signal peptide preceding the mature form of the a-amylase.Streptomycetes are gram-positive mycelial soil bacteria that obtain nutrients from macromolecular organic debris by using a wide range of hydrolytic enzymes. These enzymes include proteases, nucleases, lipases, and a variety of enzymes that hydrolyze different types of often quite complex polysaccharides (71).The last class of secreted enzyme has received considerable recent attention. Genes encoding endoglycosidase H from Streptomyces plicatus (52, 53), agarase from Streptomyces coelicolor (8, 29), a-amylase from Streptomyces hygroscopicus (26, 39), and xylanases from Streptomyces sp. strain 36a (27) and Streptomyces lividans (43) have been cloned, and the nucleotide sequences of the first three have been determined (see references 52, 12, and 26, respectively). Some of these genes are convenient for the study of gene expression and its regulation in streptomycetes, and all have potential application for the construction of expression and secretion vectors for the production of heterologous proteins in this industrially familiar genus.We recently reported the cloning of an a-amylase gene (am!) equivalent regions in other bacterial genes, including those involved in polysaccharide hydrolysis. Determination of the amino-terminal sequence of the extracellular form of the a-amylase protein enabled us to deduce the signal peptide sequence involved in the secretion process. a-Amylases are widely distributed among both procaryotes and eucaryotes. Substantial nucleotide and amino acid sequence data on these genes and proteins are now available. A previous comparison of the amino acid sequences of 11 different at-amylases was made by Nakajima et al. (46). While considerable amino acid sequence identity could be detected between six mammalian a-amylases (ranging from 80 to 90%), minimal (<10%) identity was detected between these enzymes and ax-amylases from a wide range of other species, including several bacteria, Aspergillus oryzae, and barley. Furthermore, little similarity could be detected between the a-amylases of these additional genera. In a comparison that included a-amylases from a wider variety of genera, we observed considerable amino acid sequence identity between the a-amylase of S. limosus and those of mammalian and invertebrate origin.Previous a-amylase amino acid sequence comparisons (46, 54) recognized four conserv...