Servín-González L, Jensen M R, White J, Bibb M
Department of Genetics, John Innes Centre, Colney, Norwich, UK.
Microbiology (Reading). 1994 Oct;140 ( Pt 10):2555-65. doi: 10.1099/00221287-140-10-2555.
The agarase gene (dagA) of Streptomyces coelicolor A3(2) is transcribed from four promoters that are recognized by at least three, and probably four, different RNA polymerase holoenzymes, each containing a different sigma factor. S1 nuclease protection studies revealed that transcription from all four promoters is induced by the products of agar hydrolysis and strongly repressed by glucose. Mutants deficient in glucose kinase activity were defective in glucose repression of all four promoters. Mutants were isolated or identified in which transcription from all four promoters had become inducer-independent (i.e. constitutive), establishing the existence of a repressor gene for dagA that does not appear to be located within 9 kb of the structural gene. The cloned dagA gene was also constitutively expressed in the closely related strain Streptomyces lividans, which does not normally make agarase and which presumably lacks the repressor gene. Glucose was still able to repress dagA transcription even under conditions of constitutive expression, suggesting that glucose kinase does not mediate its effect via inducer exclusion. Relative differences in the use of the four promoters were not detected during different stages of growth of surface-grown cultures, although dagA transcription appeared to peak during the production of aerial mycelium.
天蓝色链霉菌A3(2)的琼脂酶基因（dagA）由四个启动子转录，这些启动子至少被三种，可能是四种不同的RNA聚合酶全酶识别，每种全酶都含有不同的σ因子。S1核酸酶保护研究表明，来自所有四个启动子的转录都由琼脂水解产物诱导，并被葡萄糖强烈抑制。葡萄糖激酶活性缺陷的突变体在所有四个启动子的葡萄糖抑制方面存在缺陷。分离或鉴定出了突变体，其中来自所有四个启动子的转录已变得不依赖诱导物（即组成型），这表明存在一个dagA的阻遏基因，该基因似乎不在结构基因的9 kb范围内。克隆的dagA基因在密切相关的菌株淡紫灰链霉菌中也组成型表达，该菌株通常不产生琼脂酶，可能缺乏阻遏基因。即使在组成型表达的条件下，葡萄糖仍能够抑制dagA转录，这表明葡萄糖激酶不是通过诱导物排除来介导其作用的。在表面生长培养物的不同生长阶段未检测到四个启动子使用上的相对差异，尽管dagA转录似乎在气生菌丝体产生期间达到峰值。
