A mutation affecting the chloroplast coupling factor is located on 1S

A mutation designated cfr*-2018 (a lab symbol, not yet an official gene symbol) has been found that, in homozygous individuals, results in a severe reduction of the chloroplast H+-ATPase, also known as the coupling factor (CF) complex. Heterozygous individuals appear completely normal. The defect first appeared as a severe reduction in a number of leaf proteins in an SDS-PAGE analysis of seedlings from an F2 population. The relative sizes and quantities of the affected proteins and their location on thylakoid particles indicated that the CF complex was involved.

Rocket immunoelectrophoresis using antibody raised against the F1 sector of pea CF complex (supplied by Doug Randall) demonstrated that the cross-reactive F1 sector of the maize CF complex in the mutant was indeed reduced to 1-2% of normal levels. Evidence that the Fo sector is similarly reduced comes from the reduction of a prominent protein with an Mr of 5.5 Kd (on SDS-urea-gradient gels) that I think is subunit III of the Fo sector (The CF complex is composed of 2 sectors, F1 and Fo; F1 is composed of five different proteins and is located externally on the thylakoid membrane, and is attached to the Fo sector which is composed of three different proteins that are located within the membrane). No other thylakoid protein complexes appear to be affected by cfr*-2018 and thus it is unique among those mutations that are known to affect the CF complex, i.e. hcf-38 and hcf-43.

The mutant plant has slightly pale green leaves that become elongated and droopy, stops growing after the fourth leaf has expanded; and is usually dead by four to five weeks after germination. Occasionally a mutant individual will resume slow growth and produce a terminal infertile ear on a very odd-looking 30 cm tall plant.

The mutant phenotype of stunted plants and reduced CF proteins was uncovered by the B-A translocation TB-1Sb but not by TB-1Sb-2Lc. From crosses made with the TB-1Sb stock onto two different +/cfr plants the progeny segregated 22 mutants among the 133 progeny tested. The mutation was not uncovered among the progeny of crosses of the TB-1Sb-2Lc stock onto four individuals in the F2 family segregating for the mutation. The chances of missing a +/cfr plant among the four pollinated are 1 in 81. These data indicate that the cfr locus is on the short arm of chromosome 1 and possibly distal to vp5, which is located at map position 1 and is uncovered by TB-1Sb-2Lc. I'm grateful for the help of Dan England who set up and harvested the TB crosses.

Craig Echt

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