Don Robertson has given us several families of maize mutants which contain the Robertson's Mutator genetic background, and are putative photosynthetic mutants. Of the seven families we have examined, two show a marked increase in chlorophyll fluorescence, which indicates a block in photosynthetic electron transport. Figure 1 shows the fluorescence trace of an excised leaf from one of these mutants, temporarily designated hcf*-Mu-5, compared to a leaf from a wild-type plant of the same family. The wild-type trace was run at an increased sensitivity (0.1 mv/cm compared to 0.25 mv/cm for the mutant) in order to visualize the wildtype curve more easily, but the difference between the two is clear. The mutant exhibits a much higher fluorescence yield, which does not level off sharply, but curves upward to its maximum level. This probably indicates that the plastoquinone pool is functioning normally in accepting electrons from PSII. Maximum fluorescence is maintained when the plastoquinone pool is saturated and can no longer oxidize PSII, nor become oxidized itself due to a subsequent block in the electron transport chain. The shape of the Mu-5 fluorescence curve is consistent with a lesion in the b6/f complex or in photosystem 1.
LDS-polyacrylamide gel electrophoresis indicates that this is probably a photosystem I mutant. It is missing all of the major polypeptides which are associated with PSI (see Figure 2), and TMBZ staining of the gel shows the mutant to have a normal complement of cytochromes f and b6. Oxygen electrode studies, which have not yet been done, should provide further information on the exact location of the lesion. The gel profile is nearly identical to that of hcf*-50, an EMS-induced PSI mutant. Allelism tests are being conducted between hcf*-Mu-5 and hcf*-50, hcf*-49, and hcf*-44 (all deficient in PSI function).
A second mutant with increased chlorophyll fluorescence shows changes in four thylakoid polypeptides migrating in the 14-21 kilodalton region. The altered polypeptides have not been identified.
We are currently generating new photosynthetic mutants which are Mutator-induced. Since the Mu element has been cloned (Strommer et al., 1982, Nature 300:542-544) and can be used as a probe, mutants with this insertion may provide a way of locating nuclear genes for which the gene products are unknown, but which are known to play a role in the biosynthesis of the chloroplast membrane.
Marjorie Hunt and Don Miles
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