An insertion of three adenosine residues at position 116 of the psaA coding region (position 43,488 of the complete plastid chromosome sequence) which has been found not to be conserved in homologous sequences of other plant species turned out to be incorrect. The revised maize plastid chromosome sequence now consists of 140,384 base-pairs. It contains a pair of inverted repeat regions (IRA and IRB) with 22,748 base pairs each, which are separated by a small and a large single copy region (SSC and LSC) of 12,536 and 82,352 base-pairs respectively (Figure 1). With the recent identification of conserved reading frames as functional genes, the gene content of the maize plastid genome in total is 108, with 74 polypeptide-encoding genes, 30 tRNA genes and four rRNA genes (Table 1).
In addition to the 25 editing positions identified in 13 different plastid-encoded transcripts (Maier et al., see above) three new editing sites have been determined. In contrast to the C-to-U editing of ycf14 (matK)-mRNA (position 1957 of the maize plastid chromosome) which restores a highly conserved amino acid residue already found at the gene level in the liverwort Marchantia polymorpha, editing of ndhG-mRNA (position 113,177 of the maize plastid chromosome) alters coding from proline to leucine, whereas in this case a phenylalanine residue is encoded at the corresponding position in the liverwort gene. A further editing site has been detected in the non-coding 5´-untranslated region of maize ndhG transcripts (position 113,533 of the plastid chromosome).
The updated maize plastid chromosome sequence has been deposited in the EMBL data base (accession no. X86563).
Figure 1. Gene organization of the Zea mays plastid chromosome. The inverted repeat regions IRA and IRB, respectively, divide the rest of the circular chromosome into large (LSC) and small (SSC) single copy regions. Genes drawn outside the circle are transcribed clockwise. Intron-containing genes are marked by asterisks. The numbers around the circle indicate the numbers of editing sites observed in the respective genes.
Table 1. Classification of the encoded genes within the plastid DNA of Zea mays
* Intron-containing gene
§ Gene containing two introns
Ý Divided gene
# Two gene copies due to the inverted repeat
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