Chloroplast Gene Expression: Regulation at Multiple Levels

Abstract

SummaryThe plastid DNA of higher plants contains genes that primarily encode photosynthetic proteins or products required for their synthesis. Expression of plastid genes resembles prokaryotic gene expression in many aspects, reflecting the plastid’s probable origin as prokaryotic endosymbionts. Early theories suggested that plastid gene expression, like that of prokaryotes, was regulated principally at the level of transcription initiation, although further research led to competing theories regarding the relative importance of regulation at different levels. It is becoming apparent that regulation of transcription initiation is an important, but not exclusive, mechanism for control of plastid gene expression. Post-transcriptional control of transcript abundance has also emerged as an important level of regulation, with identification of nuclear-encoded factors that mediate processing, stabilization, and degradation of plastid transcripts. Translation initiation and elongation can also be specifically regulated, as can post-translational mechanisms affecting protein turnover. Thus, plastid gene expression is regulated by a combination of mechanisms that operate at multiple levels. Most of the regulatory factors affecting plastid gene expression are nuclear-encoded, illustrating the regulatory role that the nucleus plays in the control of plastid function. This allows plastid differentiation to be coordinated with the tissue-specific and developmental program of the plant. At the same time, transcription of certain nuclear-encoded plastid-localized proteins requires a plastid-generated signal; thus, expression of nuclear and plastid genes is controlled by bidirectional signaling between the two compartments. Elucidation of the mechanisms controlling the coordinated expression of the two genomes will remain a fruitful area of investigation for many years to come.