Streptomycin proteins encoded

The 30 kDa protein is encoded by nuclear DNA, and precursor protein is transported from cytoplasm to chloroplast. As the protein is located at the lumenal surface of the thylakoid, it must also cross thylakid membrane before it is incorporated into the PS II complex properly. Identification of the precursor protein has been carried out with isolation of mRNA and identification of the translation product. Transit peptide of several kilodalton is shown to be attached to the amino terminal of the mature protein. We tried to identify the precursor protein of the 30 kDa protein with Euglena cells whose chloroplast is functionally inactivated. Treatment of the cells with streptomycin under illumination induced a protein of 36 and 52 kDa which were cross-reactable with the antibody against spinach 33 kDa protein. The proteins were also detected when chloroplast-less mutant was illuminated. The 36 kDa protein is probably a precursor of the 30 kDa protein. The 52 kDa protein was dissociated into 30 kDa protein and some other protein(s) with SDS-PAGE with 8 M urea, and it is possible that we detect a complex form of the precuresor or mature 30 kDa porotein and the protein(s) which may stabilize the free 30 kda protein or assist the 30 kDa protein in transportation across the envelope and/or thylakoid membranes. Analysis is now being carried out to elucidate the characteristics of the precursor proteins. 

Hie replicase of virus Q() (an RNA-phage) consists of four subunits one is encoded by the vM genome, wliile tlie other three are the host proteins EFTu, EFTs and R.p. SI. The Fig. shows a three dimensional model for the arrangement of R.p. in the SOS-subunit. Many antibiotics act by combining with R.p., e.g. streptomycin interacts with S12, and bacterial mutants resistant to streptomycin have been shown to liave an altered S12. Erythromycin interacts with L22, spiramycin with LA. 

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