Isolation and genomic mapping of cDNAs encoding the chloroplast Rieske Fe/S protein: two unlinked genes are transcribed in maize
--Alice Barkan and Macie Walker

The chloroplast cytochrome f/b6 complex contains a single nuclear-encoded subunit, the Rieske Fe/S protein. cDNAs encoding this protein were isolated by using a monospecific antiserum to screen a cDNA expression library prepared from B73 seedling leaf RNA. Partial sequence data indicated that two classes of cDNA were obtained. The two classes are over 95% identical in the protein coding region that has been sequenced. They diverge more extensively in the 3'-untranslated region.

In high stringency Southerns (0.2 X SSC, 65 C washes) the cDNAs hybridized to two bands in DNA from the inbred lines B73, CM37, T232, Tx303 or CO159 that had been digested with each of several different enzymes . The hybridizing DNA fragments were mapped using the Burr T x CM and CO x Tx recombinant inbred populations. The two bands were unlinked. The two genes they correspond to were named ris1 and ris2. They mapped to the following locations:

    ris1: maps to chromosome 5, within 2 cM of bt1.
    ris2: maps to chromosome 4L, 9 cM distal to c2.

These results suggest that there are two closely related genes encoding the chloroplast Rieske Fe/S protein in maize, and that both genes are transcribed in leaf tissue. It seems unlikely that mutation of either gene alone would result in the loss of the cytochrome f/b6 complex. Therefore, the numerous nuclear mutations in maize that lead to the loss of the cytochrome f/b6 complex are unlikely to lie in a gene encoding the Rieske protein.

A cDNA encoding the mitochondrially localized Rieske Fe/S protein in maize was reported by Huang et al. (Proc. Natl. Acad. Sci., 1991). This cDNA is no more than 40% similar to the cDNAs encoding the chloroplast-localized protein, even in the most conserved portions of their protein coding region. Therefore the genes for the mitochondrial and chloroplast Rieske Fe/S proteins are distinct. 

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