Mapping results: nuclear mutations affecting chloroplast biogenesis
--Alice Barkan and Laura Roy

psb1 maps to chromosome 6L. psb1 is a Mu-induced mutation that causes the specific loss of photosystem II from the thylakoid membrane. A tentative map position on 8L was reported previously and is now known to be incorrect. This past summer we unambiguously mapped psb1 to 6L. Crosses of six psb1/+ plants by TB-6Lc resulted in progeny segregating pale green seedlings lacking photosystem II (specifically). In this summer's crop, crosses with other B-A stocks (including those that unmask 8L) did not unmask any mutant phenotypes.

psb2 maps to chromosome 5L. psb2 is a Mu-induced mutation that causes the specific loss of photosystem II from the thylakoid membrane. Crosses of two psb2/+ plants by TB-5La resulted in the segregation of pale green seedlings lacking photosystem II. Crosses of +/+ plants by the same B-A plants did not result in mutant progeny, nor did crosses of psb2/+ by other B-A stocks. Therefore, psb2 maps to the long arm of chromosome 5.

That psb1 and psb2 are not allelic was confirmed by the results of allelism tests, in which the two mutations complemented one another. psb1 and psb2 do not map to the same chromosome arms as two previously mapped nuclear mutations that also cause the specific loss of photosystem II (hcf3 and hcf19). Therefore, there are at least 4 nuclear genes required specifically for the biogenesis of photosystem II.

crp2 maps to chromosome 5S. crp2 (chloroplast RNA processing 2) is a Mu-induced mutation that causes defects in the metabolism of the chloroplast psaI, petA, and psbB mRNAs (unpublished results). Progeny of 14 crp2/+ plants crossed by TB-5Sc segregated small, yellow green plants with hypoploid morphology, and exhibiting the protein and RNA defects typical of crp2. Crosses of +/+ plants with the same B-A plants did not result in mutant progeny, nor did crosses of crp2/+ with other B-A translocation stocks. Therefore, the crp2 locus maps to the short arm of chromosome 5. 

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