Transcription of plastid genes and transcript accumulation were investigated in white leaves of the albostrians mutant of barley (Hordeum vulgare) and in heat‐bleached leaves of rye (Secale cereale) as well as in normal green leaves of both species. Cells of white leaves of the mutant and cells of heat‐bleached leaves bear undifferentiated plastids lacking ribosomes and, consequently, plastid translation products, among them the subunits of a putative chloroplast RNA polymerase encoded by the plastid genes rpoA, B, C1 and C2. The following results were obtained. (i) Plastid genes are transcribed despite the lack of chloroplast gene‐encoded RNA polymerase subunits. The plastid origin of these transcripts was proven. This finding provides evidence for the existence of a plastid RNA polymerase encoded entirely by nuclear genes. (ii) Transcripts of the rpo genes and of rps15, but not of genes involved in photosynthesis and related processes (psbA, rbcL, atpI‐H), were abundantly accumulated in ribosome‐deficient plastids. In contrast, chloroplasts accumulated transcripts of photosynthetic, but not of the rpo genes. (iii) Differences in transcript accumulation between chloroplasts and ribosome‐deficient plastids are due to different relative transcription rates and different transcript stability. (iv) The observed differences in transcription are not caused by an altered pattern of methylation of plastid DNA. Thus, the prokaryotic plastid genome of higher plants is transcribed by two RNA polymerases. The observed differences in transcription between chloroplasts and undifferentiated plastids might reflect different functions of the two enzymes.

Components of chlorophyll biosynthesis were investigated in the plastid-ribosome-deficient albostrians mutant of barley (Hordeum vulgare L.). Compared with green leaves, white leaves lacked chloroplast tRNA(Glu) and 16S ribosomal RNA, but contained a much higher level of the mRNA for glutamate 1-semialdehyde aminotransferase. Substantial amounts of protochlorophyllide were accumulated when the mutant was incubated in a solution of δ-aminolevulinic acid. The level of protochlorophyllide oxidoreductase mRNA (PCOR, EC 1.6.99.1.) in etiolated albostrians plants reached only about 50% of the level in wild-type plants. In addition the content of PCOR protein and the activity of chlorophyll synthetase were distinctly lower than in the wild-type. Mutant and wild-type barley seedlings which were grown under a daily light/dark regime and were therefore nonetiolated both possessed PCOR mRNA. The data presented may help explain the albino phenotype of this mutant. The results are discussed in relation to biosynthesis of tetrapyrrols in higher plants, regulation of chlorophyll biosynthesis and the action of a plastidderived signal involved in the expression of certain nuclear genes.

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