已报道大多数质体转录单元的启动子区域由原核-10和-35-样共有序列组成。然而,还表征了一些与共有元件没有同源性的启动子。本文描述了一类新型的非共有质体启动子-称为非共有II型(NC-II)启动子-影响光合和非光合质体中各自基因的低水平组成型表达。即使轻生的幼苗移至黑暗中,NC-II启动子衍生的转录物的丰度也保持不变。相比之下,与根和培养细胞的非光合质体相比,包含共有型(CT)启动子的-10和-35样元件的转录本在叶绿体中积累到高水平。此外,这些转录物的积累受到光的极大影响。质体蛋白质合成的抑制对NC-II转录物的丰度没有明显影响,而CT转录物的水平大大降低。体内标签毒素(质体转录抑制剂)处理也降低了CT转录物的水平,对NC-II转录物的积累没有明显的抑制作用。两类启动子转录本的积累,然而,当通过用环己酰亚胺体内处理抑制细胞质蛋白质合成时,减少。结果提示可能存在至少两个用于合成和/或维持质体转录物的独特系统,其以启动子类型特异性方式区分两类转录物。
The promoter regions of most plastid transcription units have been reported to consist of prokaryotic -10 and -35-like
consensus sequences. However, a few promoters with no homology to the
consensus elements have also been characterized. A novel class of non-
consensus plastid promoters--designated as non
consensus type II (NC-II) promoters--that effect low-level constitutive expression of respective genes in photosynthetic as well as non-photosynthetic plastids is described in this paper. The abundance of NC-II promoter-derived transcripts remains unaltered even when light-grown seedlings are shifted to the dark. In contrast, transcripts from -10 and -35-like elements containing
consensus type (CT) promoters accumulate to high levels in chloroplasts as compared with non-photosynthetic plastids of roots and cultured cells. Moreover, accumulation of these transcripts is greatly affected by light. The inhibition of plastid protein synthesis has no apparent effect on the abundance of the NC-II transcripts whereas levels of CT transcripts are greatly reduced. In vivo tagetitoxin (a plastid transcription inhibitor) treatment also reduces the levels of CT transcripts with no apparent inhibitory effect on the accumulation of NC-II transcripts. The accumulation of transcripts from both classes of promoters, however, is reduced when cytoplasmic protein synthesis is inhibited by in vivo treatment with
cycloheximide. The results are suggestive of the possible existence of at least two distinctive systems for the synthesis and/or maintenance of plastid transcripts which differentiate between two classes of transcripts in a promoter-type specific manner.