Ribosomal proteins S1 when associated with the 3 0 3 subunit does not interact with 16-S RNA but its binding is determined mostly by protein-protein interactions. These conclusions are based on the following data.1. Ultraviolet irradiation (1. = 254 nm) of the 30-S subunit does not result in the covalent cross-linking of S1 with 16-S RNA at irradiation doses up to 150 quanta/nucleotide, whereas the irradiation under the same conditions of S1 . polynucleotide complexes [Sl . poly(U), S1 . poly(A) and S1 . Qfl phage RNA] induces effective formation of polynucleotide-protein cross-links.2. Mild treatment of 30-S subunits lacking S-I with RNase A or with cobra venom endonuclease results in removal of 10-20% of the total nucleotide material but does not affect their sedimentation characteristics or their S1 binding capacity.3. The association of S1 with S1-depleted 30-S subunits is insensitive to aurintricarboxc\lic acid, which is known as a strong inhibitor of complex formation between S1 and polynucleotides.4. Mild trypsin treatment of S1-depleted 30-S subunits greatly reduces their S 1 binding capacity.Ribosomal protein S1 is well known to be cssential for the binding of natural mRNAs by the 30-S ribosomal subunit during the first steps of protein synthesis [l -71. The molecular aspects of S1 involvement in this process are a matter for discussion. Obviously, this problem can not be resolved until the components directly interacting with S1, both in the free 30-S subunit and in initiation complexes, have been identified. The importance of RNA-binding and RNA-melting properties of S1 for its functioning is well documented now 17-111 but the question is which RNA (16-S RNA, mRNA or both of them) directly interacts with S1 during the initiation process. The present hypotheses consider all these possibilities [4,11,12]. Thus, according to the hypothesis of Dahlberg and Dahlberg [12], S1 binds to 3'-terminal region of 16-S RNA thereby Facilitating Shine-Dalgarno interactions between this region and initiation sites of mRNA [13].Howevcr, it has recently been shown that removal of the 3'-terminal fragment of 16-S RNA does not affect the ability of the 3 0 3 subunit to bind S1 [14]. On the other hand, the importance of some 30-S ribosomal proteins for S1 binding has been demonstrated [15]. These results suggest that the association of S1 with the 30-S subunit is determined by its interactions with soinelhing other than the 3'-terminal region(s) of 16-S RNA (if at all) and/or with ribosomal proteins. Another model of S1 functioning, proposed by van Dieijen et al. [4], suggests the direct involvement of S1 in mRNA recognition and binding by the 30-S subunit. This model is also supported by finding that the RNA-melting properties of S1 are not needed for its binding with the 30-S subunit but are necessary for the ability of the latter to bind natural mRNA [7]. Nevertheless the participation of interactions between 1 6 3 RNA and S1 in S1 binding cannot be The data presented below provide evidence that the association of prot...

Mutations in sup1 and sup2 genes may cause cycloheximide-dependent growth in yeast Saccharomyces cerevisiae. Two classes of such mutants are described in the paper: 1) high temperature sensitive mutants, which do not express their sensitivity to nonpermissive temperature in the presence of cycloheximide (conditionally dependent) and 2) mutants unable to grow in the absence of the drug (true dependent). Some of the mutants of both classes express dependence toward another antibiotic - trichodermine. The binding of H(3)-labelled cycloheximide studied by equilibrium dialysis has demonstrated that both 80S ribosomes and 60S subunits isolated from conditionally dependent mutant showed a higher affinity for the drug compared to that of a parent strain. The number of binding sites per ribosome or per 60S subunit in the cycloheximide dependent mutant remains unchanged.Circular dichroism spectra of a mutant ribosomes in the presence as well as in the absence of antibiotic revealed that sup1 and sup2 mutations alter conformation of the yeast cytoplasmic ribosomes. The binding of cycloheximide to mutant ribosomes induces a conformational shift, which presumably compensates for their functional defect.

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