Twenty-two bald mutants of Streptomyces griseus were isolated and classified into four phenotypic groups, two of which showed conditional sporulation. A 3-kilobase fragment of DNA was cloned in a high-copy-number vector and detected by its ability to restore sporulation to one class of conditionally bald mutants. Analysis of subclones demonstrated that the sporulation property was contained within a 2.5-kilobase fragment. Hybridization studies and restriction analysis indicated that this DNA fragment was present in several Streptomyces species and was distinct from DNA that has been shown to complement afsA mutants of S. bikiniensis and bldA mutants of S. coelicolor.

~Phytotoxins with potato-scab-inducing activity were produced by the pathogenic Streptomyces scabies strain RB2 when grown on oatmeal agar medium or in oatmeal broth medium. Five compounds isolated from cell filtrates were identified as thaxtomin compounds 1-5, previously reported as produced by S. scabies grown on potato slices. The optimum temperature for phytotoxin production in oatmeal broth medium was 28 O C . Production of thaxtomin A, the major product, was repressed at least 130-fold when S. scabies RB2 was grown in oatmeal broth medium supplemented with 0.5 % glucose. Thaxtomin A production was also repressed by tryptophan and tyrosine, precursors of which may be involved in feedback inhibition of early steps in biosynthesis. Phytotoxins were secreted by the organism when the cells reached late exponential to early stationary phases of culture growth. The time during growth at which each thaxtomin compound was produced suggests a pathway for the latter steps in thaxtomin biosynthesis.

No abstract

When Bacteroides ovatus is grown on guar gum, a galactomannan, it produces a-galactosidase I which is different from a-galactosidase II which it produces when grown on galactose, melibiose, raffinose, or stachyose. We have purified both of these enzymes to apparent homogeneity. Both enzymes appear to be trimers and have similar pH optima (5.9 to 6.4 for a-galactosidase I, 6.3 to 6.5 for a-galactosidase II). However, a-galactosidase I has a pI of 5.6 and a monomeric molecular weight of 85,000, whereas a-galactosidase II has a pI of 6.9 and a monomeric molecular weight of 80,500. a-Galactosidase I has a lower affinity for melibiose, raffinose, and stachyose (K. values of 20.8, 98.1, and 8.5 mM, respectively) than does a-galactosidase II (Km values of 2.3, 5.9, and 0.3 mM, respectively). Neither enzyme was able to remove galactose residues from intact guar gum, but both were capable of removing galactose residues from guar gum which had been degraded into large fragments by mannanase. The increase in specific activity of a-galactosidase which was associated with growth on guar gum was due to an increase in the specific activity of enzyme I. Low, constitutive levels of enzyme II also were produced. By contrast, enzyme II was the only a-galactosidase thgt was detectable in bacteria which had been grown on galactose, melibiose, raffinose, or stachyose. Bacteroides ovatus, a gram-negative obligate anaerobe, is a member of the resident microflora of the human colon (15). This organism can ferment a variety of a-galactosides such as melibiose, raffinose, stachyose, and guar gum (7, 18), all of which contain a(1-6)-linked galactose residues (Fig. 1). All of these a-galactosides are found in human foods. Raffinose and stachyose occur naturally in beans and other vegetables, whereas guar gum is added to foods as an emulsifier. Guar gum also is being tested for possible use in reducing the rate of glucose absorption by diabetics (8, 14). Except for melibiose, these a-galactosides are not degraded and absorbed during transit through the small intestine. Thus, they may serve as natural sources of carbohydrate for B. ovatus in the colon. Although many species of colon bacteria can ferment melibiose and raffinose, only a few species can ferment galactomannans such as guar gum (18, 19). During a prelim

No abstract