Agarwal A K, Blumberg D D
Department of Biological Science, University of Maryland Baltimore County 21250, USA.
Differentiation. 1999 Jun;64(5):247-54. doi: 10.1046/j.1432-0436.1999.6450247.x.
Starvation for amino acids initiates the developmental program in the cellular slime mold, Dictyostelium discoideum [19, 20]. One of the earliest developmental events is the decline in ribosomal protein synthesis [2, 17, 29, 30]. The ribosomal protein mRNAs are excluded from polysomes with 20 min to 1 h following the removal of nutrients, and their mRNA levels decline sharply at about 9 h into the 24-h developmental cycle [28, 31, 35, 36]. It has been generally assumed that the decline in r-protein mRNA levels during late development reflected a decline in the transcription rate [12, 32, 43]. Here we demonstrate that this is not the case. The transcription rates of three ribosomal protein genes, rpL11, rpL23 and rpS9 as well as an elongation factor 1B gene have been determined during growth and development in Dictyostelium. Throughout growth and development the transcription rate of the ribosomal protein genes remains relatively constant at 0.2%-0.5% of the rate of rRNA transcription while the elongation factor 1B gene is transcribed at 0.4%-0.6% of the rRNA rate. This low but constant transcription rate is in contrast to a spore coat protein gene Psp D, which is transcribed at 6% of the rRNA rate in late developing cells. The elongation factor 1B gene appears to be co-regulated with the ribosomal protein genes both in terms of its transcription rate and mRNA accumulation. Dictyostelium has been a popular model for understanding signal transduction and the growth to differentiation transition, thus it is of significance that the regulation of ribosome biosynthesis in Dictyostelium resembles that of higher eukaryotes in being regulated largely at the post-transcriptional level in response to starvation as opposed to yeasts where the regulation is largely transcriptional [27].
氨基酸饥饿会启动细胞黏菌盘基网柄菌（Dictyostelium discoideum）中的发育程序[19, 20]。最早的发育事件之一是核糖体蛋白合成的下降[2, 17, 29, 30]。在去除营养物质后的20分钟到1小时内，核糖体蛋白mRNA从多聚核糖体中被排除，并且在24小时发育周期的大约9小时时，它们的mRNA水平急剧下降[28, 31, 35, 36]。人们普遍认为，发育后期核糖体蛋白mRNA水平的下降反映了转录速率的下降[12, 32, 43]。在此我们证明情况并非如此。我们已经测定了盘基网柄菌生长和发育过程中三个核糖体蛋白基因rpL11、rpL23和rpS9以及一个延伸因子1B基因的转录速率。在整个生长和发育过程中，核糖体蛋白基因的转录速率保持相对恒定，为rRNA转录速率的0.2% - 0.5%，而延伸因子1B基因的转录速率为rRNA速率的0.4% - 0.6%。这种低但恒定的转录速率与孢子壁蛋白基因Psp D形成对比，后者在发育后期细胞中的转录速率为rRNA速率的6%。延伸因子1B基因在转录速率和mRNA积累方面似乎都与核糖体蛋白基因共同受到调控。盘基网柄菌一直是理解信号转导以及从生长到分化转变的热门模型，因此，盘基网柄菌核糖体生物合成的调控在很大程度上类似于高等真核生物，是在转录后水平响应饥饿进行调控，这与酵母中主要是转录调控形成对比[27]，这一点具有重要意义。
