Kon T, Adachi H, Sutoh K
Department of Life Sciences, Graduate School of Arts and Sciences, University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan.
Genes Cells. 2000 Jan;5(1):43-55. doi: 10.1046/j.1365-2443.2000.00305.x.
The differentiation programme of Dictyostelium discoideum is initiated by starvation. Nutrient depletion triggers the differentiation of Dictyostelium cells through the transcriptional inactivation of some growth-phase genes, as well as through the transcriptional activation of essential genes required for the aggregation of the cells. The adenylyl cyclase (ACA) gene, acaA, is one of the earliest genes expressed following starvation. ACA produces intracellular and extracellular cAMP that drives further differentiation by inducing chemotaxis, developmental gene expression and morphogenesis of Dictyostelium cells. Although several genes have been identified as being essential for the initiation of differentiation process, such as the transcriptional activation of ACA expression, the molecular mechanisms of the growth/differentiation transition remain to be explored.
Using insertional mutagenesis, we have isolated a mutant that does not aggregate upon starvation. The disrupted gene, amiB (aggregation minus B), is predicted to encode a novel protein of 298.9 kDa. When starved, amiB- cells produced an undetectable level of cAMP. Analyses of gene expression showed that amiB- cells fail to turn off the expression of one of the growth-phase genes, cprD, and to turn on the expression of ACA following starvation. The ectopic expression of ACA from a constitutive promoter rescued the differentiation and morphogenesis of amiB- mutants. Furthermore, the ectopic expression of a putative transcriptional factor DdMyb2 or a catalytic subunit of cAMP-dependent protein kinase (PKA-C), both of which are thought to be involved in ACA expression pathway(s), also rescued the starvation-induced ACA expression and further differentiation of the amiB- mutant.
These results suggest that AmiB plays a role at the start of Dictyostelium differentiation through induction of the ACA expression which is essential for cAMP signalling.
盘基网柄菌的分化程序由饥饿引发。营养物质的耗尽通过一些生长阶段基因的转录失活以及细胞聚集所需的必需基因的转录激活来触发盘基网柄菌细胞的分化。腺苷酸环化酶（ACA）基因acaA是饥饿后最早表达的基因之一。ACA产生细胞内和细胞外的cAMP，通过诱导盘基网柄菌细胞的趋化性、发育基因表达和形态发生来驱动进一步的分化。尽管已经鉴定出几个基因对于分化过程的启动至关重要，例如ACA表达的转录激活，但生长/分化转变的分子机制仍有待探索。
利用插入诱变，我们分离出了一个在饥饿时不聚集的突变体。被破坏的基因amiB（聚集缺陷B）预计编码一种298.9 kDa的新蛋白质。饥饿时，amiB-细胞产生的cAMP水平无法检测到。基因表达分析表明，amiB-细胞在饥饿后无法关闭生长阶段基因之一cprD的表达，也无法开启ACA的表达。从组成型启动子异位表达ACA挽救了amiB-突变体的分化和形态发生。此外，推测的转录因子DdMyb2或cAMP依赖性蛋白激酶（PKA-C）的催化亚基的异位表达，这两者都被认为参与ACA表达途径，也挽救了饥饿诱导的ACA表达以及amiB-突变体的进一步分化。
这些结果表明，AmiB通过诱导对cAMP信号传导至关重要的ACA表达，在盘基网柄菌分化开始时发挥作用。
