The wording of the question suggests that it may be useful to clarify the meaning of the numbers in the designations 70S, 50S, 30S etc. These are sedimentation coefficients in Svedberg units as determined in the analytical ultracentrifuge, and are an indication of size.
All ribosomes have two subunits. The reference eubacterial ribosomes from E.coli are 70S with a large subunit of 50S and a small one of 30S. The reference eukaryotic cytoplasmic ribosomes (rat liver, say) are 80S, with large and small subunits of 60S and 40S respectively. The actually sizes (S-values) of the subunits in other eubacteria (say mycoplasma) and different eukaryotes (say yeast) may differ, but the eubacterial ribosomes share an antibiotic sensitivity (e.g. to streptomycin, erythromycin) which differs from that of eukaryotic cytoplasmic ribosomes (e.g. to cycloheximide) and so it is often convenient to refer to 70S-type ribosomes or 80S-type ribosomes, despite the size difference.
Mitochondrial and plastid ribosomes share some of the antibiotic susceptibility of eubacterial ribosomes (e.g. sensitive to streptomycin, but not to erythromycin) and their rRNA is more closely related to the latter. So they have 70S-type ribosomes with 50S-type and 30S-type subunits. However they vary greatly in size. For example, pig mitochondrial ribosomes are smaller at 55S, with large and small subunits of 39S and 28S, respectively. In contrast, yeast mitochondrial ribosomes are larger at 74S, with large and small subunits of 54S and 37S, respectively. More important, they differ from eubacteria in their ratio of RNA to protein, unlike chloroplast ribosomes, which workers in the field have classified as bacteria-like. This is shown in the table:
[Adapted from Supplementary Table S1 in Science (2014) vol 343, 1485–1489]
The striking thing about most mitochondrial ribosomes is, whether or not they have undergone a decrease in their rRNA, they have additional proteins with no analogues in bacterial ribosomes. Structural work on both mammalian and yeast mitochondrial large ribosomal subunits show that these additional proteins are associated with an altered exit tunnel for the growing polypeptides, which appears to be tailored to conduct the latter directly to the mitochondrial membrane.
Answers (depending on intent of question)
- Yes, mitochondrial and plastid ribosomes have large and small subunits.
- No, their sizes are not 50S and 30S, but vary greatly.
- Yes, mitochondria and plastids have 70S-like ribosomes, rather than 80S-like ribosomes, but these have evolved considerably from their eubacterial precursors.
Some of my assertions are based on recent papers and earlier work referenced in them. Also access to them may not be free outside university libraries. Apologies for that — if I find more accessible sources I will add them later.
- Mammalian mitochondrial ribosomes: Nature (2014) vol 505, pp. 515–521.
- Yeast mitochondrial ribosomes: Science (2014) vol 343, 1485–1489.
- Spinach Chloroplast ribosomes: Proc.Nat.Acad.Sci. (2007) vol 104, pp. 19315–19320.
- Antibiotics and ribosomes (review): EMBO Reports (2001) vol 2, pp. 318–323.