The time course of polysome formation was studied in a long-term wheat germ cell-free translation system using sedimentation and electron microscopy techniques. The polysomes were formed on uncapped luciferase mRNA with translation-enhancing 5′ and 3′ UTRs. The formation of fully loaded polysomes was found to be a long process that required many rounds of translation and proceeded via several phases. First, short linear polysomes containing no more than six ribosomes were formed. Next, folding of these polysomes into short double-row clusters occurred. Subsequent gradual elongation of the clusters gave rise to heavy-loaded double-row strings containing up to 30–40 ribosomes. The formation of the double-row polysomes was considered to be equivalent to circularization of polysomes, with antiparallel halves of the circle being laterally stuck together by ribosome interactions. A slow exchange with free ribosomes and free mRNA observed in the double-row type polysomes, as well as the resistance of translation in them to AMP-PNP, provided evidence that most polysomal ribosomes reinitiate translation within the circularized polysomes without scanning of 5′ UTR, while de novo initiation including 5′ UTR scanning proceeds at a much slower rate. Removal or replacements of 5′ and 3′ UTRs affected the initial phase of translation, but did not prevent the formation of the double-row polysomes during translation.

Highly efficient DNA synthesis without template and primer DNAs occurs when N.BspD6I DNA nickase is added to a reaction mixture containing deoxynucleoside triphosphates and the large fragment of Bst DNA polymerase. Over a period of 2 h, virtually all the deoxynucleoside triphosphates (dNTPs) become incorporated into DNA. Inactivation of N.BspD6I nickase by heating inhibits DNA synthesis. Optimal N.BspD6I activity is required to achieve high yields of synthesized DNA. Electron microscopy data revealed that the majority of DNA molecules have a branched structure. Cloning and sequencing of the fragments synthesized demonstrated that the DNA product mainly consists of multiple hexanucleotide non-palindromic tandem repeats containing nickase recognition sites. A possible mechanism is discussed that addresses template-independent DNA synthesis stimulated by N.BspD6I nickase.

MS2 phage RNA-directed synthesis of an N-terminal polypeptide of the phage coat protein on Escherichia coli 70 S ribosomes was initiated in a cell-free system with the N-dinitrophenyl derivative of methionyltRNAp and performed in the absence of tyrosine, lysine, cysteine and methionine. As a result, the translating ribosomes carried peptides up to 42 amino acid residues in length with the dinitrophenyl hapten at the N-ends. Using the immune electron microscopy technique the positions of the nascent peptide N-ends on the 70 S ribosomes have been visualized. It has been found that (i) the N-ends of nascent peptides of these lengths are accessible to antibodies, (ii) the exit site of a nascent peptide is the pocket between the base of the central protuberance and the Ll ridge on the 50 S subunit, i.e. presumably its peptidyl transferase center, and (iii) the further pathway of a nascent peptide seems to proceed along the groove on the external surface of the 50 S subunit.Ribosome; MS2 phage; RNA; Nascent polypeptide; Immune electron microscopy

No abstract

Interaction of delta-endotoxin and its proteolytic fragments with phospholipid vesicles was studied using electron microscopy, scanning microcalorimetry, and limited proteolysis. It was shown that native protein destroys liposomes. The removal of 4 N-terminal alpha-helices and the extreme 56 C-terminal amino acid residues did not affect this ability. The results obtained by limited proteolysis of delta-endotoxin bound to lipid vesicles show essential conformational changes in three or four N-terminal helices and in the C-terminal region. The calorimetric method used in this study provides a unique possibility for the validation of existing models of protein binding and for a more accurate determination of the regions where conformational changes take place. It was found that the binding of the protein to model liposomes does not alter its structure in the regions starting with the fourth alpha-helix of domain I. This can be concluded from the fact that the activation energy of denaturation of the protein remains unchanged upon its binding to the phospholipid membranes. A new structural model has been proposed which agrees with the data obtained.