A nuclear mutation in maize blocks the processing and translation of several chloroplast mRNAs and provides evidence for the differential translation of alternative mRNA forms

EMBO J. 1994 Jul 1;13(13):3170-81.

Abstract

A mutant designated crp1 (chloroplast RNA processing 1) was identified in a screen for transposon-induced maize mutants with defects in chloroplast gene expression. crp1 is a recessive, nuclear mutation that causes the loss of the cytochrome f/b6 complex and a reduction in photosystem I. The molecular basis for these protein losses is unique relative to previously described mutants with defects in organelle gene expression; it involves defects in the metabolism of two organellar mRNAs and in the translation of two organellar proteins. Mutants lack the monocistronic forms of the petB and petD mRNAs (encoding cytochrome f/b6 subunits), but contain normal levels of their polycistronic precursors. Pulse-labeling experiments revealed normal synthesis of the petB gene product, but a large decrease in synthesis of the petD gene product. These results suggest that petD sequences are more efficiently translated in a monocistronic than in a polycistronic context, thereby providing evidence that the elaborate RNA processing typical of chloroplast transcripts can play a role in controlling gene expression. Structural predictions suggest that the petD start codon lies in a stable hairpin in the polycistronic RNA, but remains unpaired in the monocistronic transcript. Thus, processing to a monocistronic form may increase translational efficiency by releasing the translation initiation region from inhibitory interactions with upstream RNA sequences. Synthesis of a third cytochrome f/b6 subunit, encoded by the petA gene, was undetectable in crp1, although its mRNA appeared unaltered. Two mechanisms are consistent with the simultaneous loss of both petA and petD protein synthesis: the translation of the petA and petD mRNAs might be coupled via a mechanism independent of crp1, or the crp1 gene may function to coordinate the expression of the two genes, which encode subunits of the same complex.

Publication types

  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Alternative Splicing
  • Base Sequence
  • Binding Sites
  • Chloroplasts / metabolism
  • Cytochrome b Group / biosynthesis
  • Cytochrome b Group / genetics
  • Cytochrome b6f Complex
  • DNA
  • Genes, Plant / genetics
  • Genes, Recessive
  • Molecular Sequence Data
  • Mutation*
  • Photosynthetic Reaction Center Complex Proteins / biosynthesis
  • Photosynthetic Reaction Center Complex Proteins / genetics
  • Photosystem I Protein Complex
  • Plant Proteins / biosynthesis
  • Plant Proteins / genetics
  • Polyribosomes / metabolism
  • Protein Biosynthesis*
  • RNA Precursors / metabolism
  • RNA Processing, Post-Transcriptional*
  • RNA, Messenger / metabolism*
  • Transcription, Genetic
  • Zea mays / genetics*

Substances

  • Cytochrome b Group
  • Photosynthetic Reaction Center Complex Proteins
  • Photosystem I Protein Complex
  • Plant Proteins
  • RNA Precursors
  • RNA, Messenger
  • DNA
  • Cytochrome b6f Complex