Can a non-Mendelian Mutation Affect Both Chloroplast and Mitochondrial Ribosomes?

Abstract

Chloroplast ribosomes isolated from a spectinomycin-resistant mutant (spr-1-27- Mitochondrial structure and function of the mutant are not affected by the antibiotic, whereas chloroplast structure and function are. Apparently, the mitochondrion, rather than the chloroplast, of this mutant is resistant to spectinomycin in vivo. We hypothesize that the gene product of the spr locus is a protein common to both chloroplast and mitochondrial ribosomes. The mutant gene product, in vivo, confers resistance on mitochondrial, but not chloroplast, ribosomes. We suppose that the mutant spr protein loosely attaches to chloroplast ribosomes in vivo, so that the antibiotic is bound and blocks protein synthesis, but it dissociates during isolation, resulting in loss of the binding site.

Etiolated cells of Chlamydomonas reinhardtii: choice material for characterization of mitochondrial membrane polypeptides.

“…This will be of great importance in reevaluating the question of a possible interdependence of mitochondrial and chloroplast translation. Indeed, a number of previous observations suggest that in C. reinhardtii, mitochondrial protein synthesis depends on chloroplast protein synthesis (35)(36)(37)(38), a feature that could be unique to plant cells that have three distinct but interacting genetic compartments.…”

mRNAs for two ribosomal proteins are preferentially translated in the chloroplast of Chlamydomonas reinhardtii under conditions of reduced protein synthesis

“…1, Table 1). Preferential accumulation of other chloroplast-synthesized ribosomal proteins must also occur in these mutants, since the large subunits of chloroplast ribosomes formed by the cr-1 mutant [5] and the ribosome monomers made by the spr-u-1-27-3 mutant grown in the presence of spectinomycin [4] contain most, if not all, of the ribosomal proteins present in wild-type chloroplast ribosomes, including those made in the chloroplast (data not shown). On the other hand, the severe deficiency of Rubisco LSU and the alpha subunit of CF 1 very likely extends to other non-ribosomal chloroplast proteins, especially those in the thylakoid membrane, as indicated by the reduction in electron transport (Hill reaction) activity and the severe deformation in the stacking of the thylakoids observed previously in these mutants [4, 5, 7, i3].…”

“…Furthermore, our data show that the deficiency of the non-ribosomal proteins Rubisco LSU and the alpha subunit of CF 1 results from reduced rates of synthesis rather than turnover. Deficiency in the synthesis of other non-ribosomal proteins is also expected, since the cr-1 mutant has a very low level of chloroplast ribosome monomers [5], and the spru-I-27-3 mutant accumulates chloroplast ribosomes which are only slightly resistant to spectinomycin in vivo [4] and in vitro [I, 7].…”

“…The spectinomycin resistant mutant spr-u-l-27-3 has a GC to AT transition at position 1137 in the 16S rDNA of the chloroplast genome [14]. When grown in the presence of spectinomycin with acetate as a carbon source, this mutant accumulates wild-type levels of chloroplast ribosomes, but exhibits the same symptoms of reduced chloroplast protein synthesis as cr-1, including a deficiency in Rubisco activity, reduced electron transport (Hill reaction) activity, and abnormal thylakoid stacking [4][5][6][7]. This same syndrome of photosynthetic defects is seen in all chloroplast ribosomedeficient mutants so far characterized [6,13].…”

Translational Regulation of Chloroplast Genes

“…As found earlier for r-protein L-2 (9), we also observe that synthesis of r-proteins S-7 and S-12 preferentially continues under conditions of reduced chloroplast protein synthesis. Furthermore, all or most of the remaining chloroplast-encoded r-proteins must also continue to be synthesized, since the ribosome monomers made by the spr-u-1-27-3 mutant grown in the presence of spectinomycin accumulate in nearly normal amounts (49). exist of nuclear-encoded trans-acting proteins that bind to the 5Ј-UTRs of individual chloroplast and mitochondrial mRNAs in a gene-specific manner (1), only one universal binding protein (p40) has been reported for yeast mitochondria and none, to the best of our knowledge, for the chloroplast 5Ј-UTRs (16).…”