Regulation of Plastid Gene Expression

Abstract

Plastids are known to contain prokaryote-like genome within a eukaryotic cell. The plastid genome is a circular molecule of about 150 kb and codes for functions related to genetic system of plastids as well as those related to photosynthesis. Due to limited coding potential of plastid genome, its products must interact with nuclear-encoded products to establish a functional form of plastids. It has been found that in addition to well-defined prokaryote-like promoter sequences, plastid genes also contain some novel regulatory elements. These interact with multiple forms of RNA polymerase(s) and other regulatory proteins which control expression of plastid genes in light-dependent and development-specific manner. These stimuli seem to exert their effect by multi-step signal transduction chain at transcriptional, post-transcriptional and translational levels. While post-transcriptional steps include processing of polycistronic RNA, editing and stability, translation is controlled at the steps of chain elongation and protein turn-over which in turn depends on chlorophyll. The components involved in these processes include a plethora of proteins whose identity is being revealed by recent investigations. It seems that important regulatory steps are dependent on redox state and phosphorylation status. Knowledge is expected to accumulate rapidly keeping in view concerted efforts in the area of signal transduction and gene expression and development of techniques for transforming plastids.