Effects of insulin on the fine structure of hepatocytes from winter-acclimatized carps: studies on protein synthesis

Male and female winter-acclimatized carps were injected with insulin. This treatment resulted in a sharp decrease in the liver glycogen content. Although an increase in the ribosomal RNA level was also observed, a cell-free system obtained from the hormone-treated fish exhibited less amino acid incorporation activity as compared to the control fish. However, polysomes from insulin-treated fish exhibited a higher amino acid incorporating activity when a soluble fraction of untreated winter carps was used. Insulin induced a profound change in the cytoarchitecture of the winter carp hepatocyte. The cytoplasm and nuclei showed all the features of the summer carp liver cell. The nucleolar components were totally intermingled suggesting a high rate of gene expression as in the case of the summer-acclimatized fish.     

Tissue expression of glandular kallikrein and its response to 17 beta-estradiol in the acclimatized carp

Cyprinus carpio skeletal muscle kallikrein was isolated to apparent homogeneity, and a polyclonal antiserum against the purified protein was generated. Glandular kallikrein expression and tissue distribution were assessed using both Western blots and immunohistochemistry. A 39-kDa protein was detected in skeletal muscle, the gill, kidney, and pituitary gland, where an additional 72-kDa immunoreactive band was observed. Immunohistochemistry revealed immunoreactive kallikrein in the intermuscle tissue, epithelial gill cells, apical portion of distal and proximal tubular cells in the kidney, mucus and epithelial cells of the skin, intestinal tube, and prolactin-producing cells of the pituitary gland. In addition, the effect of 17beta-estradiol on kallikrein expression was analyzed in three different tissues of winter- and summer-acclimatized male carps. A 2.5-fold (p<0.05) increase in kallikrein immunoreactivity due to estrogen treatment was observed in winter-acclimatized carp muscle, but not in summer-acclimatized fish. In contrast, the gill responded differently, since a 2-fold (p<0.05) increase was found only in summer-acclimatized carps. Kallikrein immunoreactivity in the kidney increased both in summer- (2.5 fold) and in winter-acclimatized carps (1.5 fold). The signals obtained demonstrate the existence of tissue-specific variable responses to estrogen treatment in vivo, between winter and summer-acclimatized carp.     

Translational regulation by ribosomal protein S8 in Escherichia coli: structural homology between rRNA binding site and feedback target on mRNA.

It has been previously shown that ribosomal protein synthesis in Escherichia coli is regulated at the level of translation by certain key ribosomal proteins. In the spc operon, S8 regulates the expression of L5 and some of the subsequent genes, while the first two genes (L14 and L24) are regulated independently. We therefore determined the DNA sequence at the junction of the L24 and L5 genes, which corresponds to the putative feedback target for S8. We show that there is a striking homology between the structure of the mRNA for this region and the known binding site for S8 on 16S rRNA. These results support the theory that the regulation of ribosomal protein synthesis is based on competition between rRNA and mRNA for regulatory ribosomal proteins.     

Protein-rRNA binding features and their structural and functional implications in ribosomes as determined by cross-linking studies.

We have investigated protein-rRNA cross-links formed in 30S and 50S ribosomal subunits of Escherichia coli and Bacillus stearothermophilus at the molecular level using UV and 2-iminothiolane as cross-linking agents. We identified amino acids cross-linked to rRNA for 13 ribosomal proteins from these organisms, namely derived from S3, S4, S7, S14, S17, L2, L4, L6, L14, L27, L28, L29 and L36. Several other peptide stretches cross-linked to rRNA have been sequenced in which no direct cross-linked amino acid could be detected. The cross-linked amino acids are positioned within loop domains carrying RNA binding features such as conserved basic and aromatic residues. One of the cross-linked peptides in ribosomal protein S3 shows a common primary sequence motif--the KH motif--directly involved in interaction with rRNA, and the cross-linked amino acid in ribosomal protein L36 lies within the zinc finger-like motif of this protein. The cross-linked amino acids in ribosomal proteins S17 and L6 prove the proposed RNA interacting site derived from three-dimensional models. A comparison of our structural data with mutations in ribosomal proteins that lead to antibiotic resistance, and with those from protein-antibiotic cross-linking experiments, reveals functional implications for ribosomal proteins that interact with rRNA.     

Lack of complete cooperativity of ribosome assembly in vitro and its possible relevance to in vivo ribosome assembly and the regulation of ribosomal gene expression.

Earlier studies have shown that the reconstitution of Escherichia coli 50S as well as 30S ribosomal subunits from component rRNA and ribosomal protein (r-protein) molecules in vitro is not completely cooperative and binding of more than one r-protein to a single 16S rRNA (or 23S rRNA) molecule is required to initiate a successful 30S (or 50S) ribosome assembly reaction. We first confirmed this conclusion by carrying out 30S subunit reconstitution in the presence of a constant amount of 16S rRNA together with various amounts of total 30S r-proteins (TP30) and by analyzing the physical state of reconstituted particles rather than by assaying protein synthesizing activity of the particles as was done in the earlier studies. As expected, under conditions of excess rRNA, the efficiency of 30S subunit reconstitution per unit amount of TP30 decreased greatly with the decrease in the ratio of TP30 to rRNA, indicating the lack of complete cooperativity in the assembly reaction. We then asked the question whether the cooperativity of ribosome assembly is complete in vivo. We treated exponentially growing E coli cells with low concentrations of chloramphenicol which is known to inhibit protein synthesis without inhibiting rRNA synthesis, creating conditions of excess synthesis of rRNA relative to r-proteins. Several concentrations of chloramphenicol (ranging from 0.4 to 4.0 micrograms/ml) were used so that inhibition of protein synthesis ranged from 40 to 95%. Under these conditions, we examined the synthesis of RNA, ribosomal proteins and 50S ribosomal subunits as well as the synthesis of total protein. We found that the synthesis of 50S subunits was not inhibited as much as the synthesis of total protein at lower concentrations of chloramphenicol, but the degree of inhibition of 50S subunit synthesis increased sharply with increasing concentrations of chloramphenicol and was in fact greater than the degree of inhibition of total protein synthesis at chloramphenicol concentrations of 2 micrograms/ml or higher. The inhibition of 50S subunit synthesis was significantly greater than the inhibition of r-protein synthesis at all chloramphenicol concentrations examined. These data are consistent with the hypothesis that the cooperativity of ribosome assembly in vivo is also not complete as is the case for in vitro ribosome reconstitution, but are difficult, if not impossible, to explain on the basis of the complete cooperativity model.(ABSTRACT TRUNCATED AT 400 WORDS)     

Requirement for C-terminal extension to the RNA binding domain for efficient RNA binding by ribosomal protein L2

Ribosomal protein L2 is a primary 23S rRNA binding protein in the large ribosomal subunit. We examined the contribution of the N- and C-terminal regions of Bacillus stearothermophilus L2 (BstL2) to the 23S rRNA binding activity. The mutant desN, in which the N-terminal 59 residues of BstL2 were deleted, bound to the 23S rRNA fragment to the same extent as wild type BstL2, but the mutation desC, in which the C-terminal 74 amino acid residues were deleted, abolished the binding activity. These observations indicated that the C-terminal region is involved in 23S rRNA binding. Subsequent deletion analysis of the C-terminal region found that the C-terminal 70 amino acids are required for efficient 23S rRNA binding by BstL2. Furthermore, the surface plasmon resonance analysis indicated that successive truncations of the C-terminal residues increased the dissociation rate constants, while they had little influence on association rate constants. The result indicated that reduced affinities of the C-terminal deletion mutants were due only to higher dissociation rate constants, suggesting that the C-terminal region primarily functions by stabilizing the protein L2-23S rRNA complex.