L11 cistron (Yates & Nomura, 1981). It follows that the attachment sites of the repressor proteins should be located at the origins of the cistrons from which the repression of the sequential translation begins in polycistronic mRNA. Correspondingly, for protein S7 the operator site should lie somewhere between the cistrons of proteins S12 and S7 or at the origin of the S7 cistron; for protein S4 the operator should be located before or at the beginning of the S13 cistron; and for protein L1 this site should be before the L11 cistron. Continuing this line of argument, the site of the repressor action for protein L4 should be located prior to or at the beginning of the S10 cistron (Yates & Nomura, 1980); for protein L10, prior to or at the beginning of its own cistron (Yates et al., 1981; Johnsen et al., 1982); and for protein S8, between the cistrons of proteins L24 and L5 or at the beginning of the L5 cistron (Dean et al., 1981).

It is known that proteins S4, S7, S8, L1, and L4 play an important part in ribosomal structure and

S10 L3 L4 L23 L2 L22 L19 S3 L16 L29 S17 5' _I_I_iv vi_I_I__I_I_I_I_I_I_ 3'

L14 L24 L5 S14 S8 L6 L18 S5 L30 L15 5' _I_I_I_I_iv VI_I_I_I_I_I_3'

Figure 16.7. Schematic representation of the sequential arrangement of ribosomal protein cistrons along polycistronic mRNA chains (D. Dean & M. Nomura, Proc. Natl. Acad. Sci. USA 77, 3590-33594, 1980; M. Nomura, S. Jinks-Robertson & A. Miura, in "Interaction of Translational and Transcriptional Controls in the Regulation of Gene Expression", M. Grunberg-Manago & B. Safer, eds., p.p. 91-104, Elsevier, New York). a, p and p' are the cistrons coding for the corresponding subunits of RNA polymerase. The origin of the arrow under the sequence designates the point of action of a repressor protein; the arrow extends to the cistron subject to the control. The circled ribosomal proteins serve as repressors; they bind to the regions of the polycistronic mRNAs corresponding to the origins of the arrows.

self-assembly: they are core proteins that bind tightly to specific sites on the ribosomal RNA (see Section 7.5). The attachment sites of these proteins on ribosomal RNA have been identified (see Fig. 7.6). There are all grounds to believe that these ribosomal proteins, playing the role of repressors, bind to mRNA through the same RNA-binding centers that participate in the binding to ribosomal RNA. Then, it may be expected that the structures of the RNA regions binding a given ribosomal protein should be similar in ribosomal RNA and in mRNA. Indeed, the comparisons of primary and predicted secondary structures of the protein-binding site on the ribosomal RNA and at the operator regions of mRNAs have demonstrated, at least in some cases, their similarity: the operator regions of mRNAs may mimic the protein-binding sites

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