A mutation isolated from among M2 progeny mutagenized with Robertson's Mutator (MNL 60:46) has been found to be allelic to the EMS-induced mutation responsible for the photosynthetic mutant hcf3 (Leto and Miles, Pl. Physiol. 66:18). hcf3 has been described as "one of the most extensively characterized PSII-deficient mutants in higher plants" (Somerville, Ann. Rev. Pl. Physiol. 37:483). It exhibits a high level of chlorophyll fluorescence, abnormal fluorescence induction kinetics and lacks electron transport activity through PSII. Polypeptides associated with PSII are severely reduced or missing when membrane proteins are examined by LDS-polyacrylamide gel electrophoresis. The hcf3 lesion has been located on chromosome 1S but no information about the nucleotide sequence of the gene or about its product is available.

The isolation of a mutation allelic to hcf3 which is tagged with the Mu transposon opens the door to the cloning and characterization of the gene and its translation product. Ultimately this may lead to a better understanding of the nature of the mutation in particular and of the structure and function of PSII in general. We are currently considering the most appropriate method of cloning the Mu-tagged gene.

Allelism was tested by crossing pollen from M2 progeny of the Mu mutagenesis onto the ears of progeny of the original hcf3 isolate. Eight of 50 progeny of this cross exhibited high fluorescence and the expected fluorescence induction kinetics. The genotypes of both parents of this cross were confirmed. Three other crosses between siblings of this parent and plants bearing hcf3 in other backgrounds also produced progeny with hcf3 characteristics. For these crosses, only the Mu parents' genotypes were confirmed.

Bill Cook and Don Miles

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