Institute of Molecular Biology

A Mu-induced mutation that blocks the stable assembly of the photosystem II complex

--Rodger Voelker and Alice Barkan

hcf134 is one of twenty-seven hcf mutants that we have isolated from the Mu stocks propagated by the Hake and Freeling groups (see contribution by Barkan et al.). Its phenotype is unique among this group of mutants in that it lacks only the photosystem II (PSII) core complex; other chloroplast proteins accumulate normally. Since there are no nuclear-encoded subunits of the PSII core complex, the Hcf134 locus appears to play an important role in controlling the expression of the chloroplast genes encoding the missing subunits, or in mediating the assembly of the complex.

We have found that hcf134 is a recessive mutation. Mutant seedlings are slightly paler than normal, and exhibit small dark-grown sectors that presumably arise as a result of somatic excision of a transposon from the Hcf134 gene. They are unable to photosynthesize and thus die at the three to four leaf stage, when seed reserves are exhausted.

Western analysis showed that all of the major chloroplast proteins accumulate normally except for the components of the PSII core. The products of the psbA, psbB, psbC, and psbD genes accumulate to less than 5% of wild type levels, while the psbE product accumulates to approximately 20% of normal levels. The mRNAs encoding these proteins are normal in size and abundance, as revealed on Northern blots. Sucrose gradient fractionation of total leaf lysates demonstrated that these mRNAs are associated with polysomes in a normal fashion, suggesting that the rate of translation is probably unaffected by the mutation. Pulse-labelling experiments in vivo and in isolated chloroplasts confirmed that the missing PSII proteins are in fact synthesized at near wildtype rates. Although the stability of these proteins has not been carefully determined, it appears that they have a half life of about 15 minutes in mutant leaves. These results reveal that the hcf134 mutation prevents the stable assembly, rather than the synthesis of the PSII core complex.

hcf3 is an EMS-induced mutation that also exhibits the specific loss of the PSII core complex (Leto and Miles, Plant Physiol. 66: 18-24, 1980). However, we have found that hcf134 and hcf3 are not allelic. We have used B-A translocations to map hcf134 to the long arm of chromosome 8.

The exact role of the hcf134 gene product is unknown. We speculate that it may play a role in the insertion of the PSII components into the thylakoid, in their assembly into a complex, in their post-translational modification, or in prosthetic group biosynthesis.

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