Streptomyces avermitilis
A Microbial Biorealm page on the genus Streptomyces avermitilis
Classification
Bacteria; Actinobacteria; Actinobacteridae; Actinomycetales; Streptomycineae; Streptomycetaceae; Streptomyces; Streptomyces avermitilis
Species
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NCBI: Taxonomy |
Streptomyces avermitilis
Description and significance
Streptomyces avermitilis was first isolated in 1979 at Kitasato Institute from a soil sample collected at Kawana, Ito City, Shizuoka Prefecture, Japan. It was sent to Merck Sharp & Dohme Research Laboratories for screen testing. (1)
This particular Streptomyces species dwells in terrestrial soils and has a brownish-gray spore mass. The spores are spherical (as opposed to oval) with a smooth spore surface and come in chains of more than 15. The sporophores form spiral side branches on aerial mycelia. (1)
S. avermitilis is an important species to have its genome sequenced because it produces certain secondary metabolites, called avermectins, that have antihelmintic and insecticidal properties. (2)
Genome structure
The chromosome of Streptomyces avermitilis contains over 9 million base pairs (9.02-Mb), putting it at the top as one of the largest bacterial genomes sequenced as of yet. It also has a higher GC content (70.7%) than almost any other organism, making the S. avermitilis chromosome unique in its size and structure (3). This linear chromosome contains 7,574 open reading frames (ORFs), as well as unique terminal-inverted repeats at both ends that bind terminal proteins. Of the 7,574 ORFs, 60.2% (or 4,563) encode funtional proteins and about 35% (or 2,663) cluster into 721 paralogous families. Of these 721, two main gene families are represented -- one relating to membrane-spanning components of ABC transporters, and the other relating to two-component transcriptional regulator systems. These results suggest that at least a third of all S. avermitilis genes may have emerged as a result of gene duplication during evolution (4).
Comparing the genomes of S. avermitilis and S. coelicolor A3(2) revealed a 6.5-Mb highly conserved internal region where all the essential genes are located (SAV1625-7142 in S. avermitilis). This region is structurally similar to other circular bacterial chromosomes, implying that this 6.5-Mb internal region may be the underlying backbone of all Streptomyces chromosomes and may have evolved from an ancestor common to all bacteria with circular chromosomes. Conversely, there are also variable, less conserved regions found near both telomeres. More than half the genes related to secondary metabolism (17 out of 30) were found in subtelomeric regions, while no known essential genes were found there (4).
S. avermitilis also contains the plasmid SAP1. SAP1 has 96 ORFs, 34.4% (or 33) of which encode funtional proteins (4).
Overall, the gene content of S. avermitilis suggests that its genome may have evolved by acquisition of novel gene functions that aided in its adaption to the intense competition, unpredictable fluctuation of nutrients, and extremely variable physical conditions of soil environments (4).
Cell structure and metabolism
One of the unique characteristics of S. avermitilis is its production of antiparasitic secondary metabolites, called avermectins. This production is regulated by several parameters, one of them being glucose metabolism. Avermectin formation is suppressed by the addition of glucose at an early stage of fermentation, but not when glucose is added at a later stage. (5)
Most organisms metabolize glucose rapidly, while S. avermitilis digests glucose slowly. There are two major pathways of glucose metabolism -- the Embden-Meyerhof pathway and the pentose phosphate pathway. Metabolites of glucose are then further metabolized in the tricarboxylic acid cycle. The activities of certain enzymes in these metabolic pathways, such as glucose-6-phosphate dehydrogenase, phosphofructokinase, and citrate synthase, are not effected by higher glucose concentrations. However, the activity of 6-phosphogluconate dehydrogenase, a rate-limiting step in the pentose phosphate pathway, is significantly reduced when glucose is overly present at an early stage. (5)
Since glucose is the best carbon source for avermectin production, the amount of glucose effects the rate of avermectin production, as well as the activity of 6-phosphogluconate dehydrogenase. Though avermectin formation is suppressed by the addition of glucose at an early stage of fermentation, the production rate is restored at a later stage as the activity of 6-phosphogluconate dehydrogenase is also restored. (5)
Ecology
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Pathology
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Application to Biotechnology
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Current Research
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References
Edited by Jennifer Woods, student of Rachel Larsen
