Mutable cytochrome f/b6 mutant isolated in Spm background

We report the isolation of a mutable nuclear pale green recessive maize mutant, pg*-m1998, from a line with autonomous Spm. Leaf chlorophyll fluorescence induction kinetics indicated that the mutant is blocked on the reducing side of photosystem II. Analysis of thylakoid polypeptides by lithium dodecylsulfate polyacrylamide electrophoresis, followed by sequential staining for heme and polypeptides, revealed that the mutant had greatly reduced amounts of a polypeptide that stained positively for heme and corresponded to cytochrome f (35-37 kD apparent molecular weight). No cytochrome b6 was detected by heme-staining. No other differences were noted in the polypeptide composition by these criteria.

Western blot analyses confirmed that the mutant had trace levels of cytochrome f and also showed that the mutant had reduced (50-70%) levels of subunit 4 and Rieske FeS polypeptide. We had no specific antisera for cytochrome b6. The antisera were kindly provided by W. Taylor, Berkeley, CA. Of the four polypeptides in the cytochrome fb6 complex, only one, the Rieske FeS polypeptide, is nuclear encoded. Two nonallelic nuclear mutations (hcf2, hcf6) have been reported in maize that inhibit assembly of this complex (Metz et al., Plant Physiol. 73:452-459, 1983). We plan to test allelism of the new locus with hcf2 and hcf6. The recent cloning of the spinach structural gene for the pg* -m1998 Rieske FeS polypeptide should permit assessment of linkage with this locus (loci) if the maize and spinach gene(s) share adequate homology. Work is in progress to test whether pg*-m1998 is under Spm control, and to ascertain whether the other heme protein of thylakoids, cytochrome b559, is affected by the mutation.

Leonard Rosenkrans, Mary Polacco and Craig Echt
 
 


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