Multiple transcripts for higher plantrbcL andatpB genes and localization of the transcription initiation site of therbcL gene.
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We have compared therbcL andatpB transcription units from spinach, maize, and pea. In most cases multiple transcripts were found for a given chloroplast gene. The 5' termini of these transcripts were determined by S1 nuclease protection and primer extension analyses. TherbcL transcripts have 5' termini 178-179 and 64 nucleotides (spinach), 300 and 59-63 nucleotides (maize), and 178 and 65 nucleotides (pea) upstream from their respective protein coding regions. TheatpB transcripts have 5' termini (453-454, 272-273, 179, and 99 nucleotides (spinach), 298-302 nucleotides (maize), and 351-355 nucleotides (pea) upstream from their respective protein coding regions. The intergenic distance between therbcL andatpB genes is relatively constant (152 to 157 base pairs) among the three chloroplast genomes. In spinach, maize, and pea, the 80 base pairs surrounding the 5' end of therbcL gene (40 base pairs) have 85% sequence homology. Similarly, the 60 base pairs preceding theatpB gene have 48% sequence homology. Both genes have '-10' and '-35' regions that resemble the prokaryotic consensus promoter sequence. The larger, but not smaller,rbcL transcripts from spinach and pea can be labeled with alpha-(32)P-GTP by guanylyltransferase. These data suggest that DNA sequences 178-179 (spinach), 300 (maize), and 178 (pea) base pairs before therbcL protein coding regions represent sites of transcription initiation. The sequences 59-65 base pairs before therbcL protein coding regions may correspond to sites of RNA cleavage.
