Judith Ilan

Stage-specific initiation factors for protein synthesis during insect development☆

Abstract In insects, as in bacteria, the smaller (40 S) ribosomal subunit binds messenger RNA during initiation of protein synthesis. An 80 S ribosomal unit is formed by association of free 40 S and 60 S subunits. Formation of the complete initiation complex requires GTP, aminoacyl-tRNA, protein initiation factors and messenger RNA. The complex sediments as an 80 S band on sucrose gradient. Protein initiation factors are extracted from unwashed ribosomes and appear to be able to discriminate between messenger RNAs obtained from different stages of development. They promote formation of the 80 S complex only when messenger RNA is extracted from the same stage of development, providing a mechanism for control of protein synthesis by which ribosomes can select the messenger RNA to be translated. Two possibilities have been proposed to explain this phenomenon: (1) that a group of messenger RNAs from a given stage of development may have a specific sequence of nucleotides preceding the AUG codon. This sequence is recognized by a stage-specific element of the initiation machinery; (2) and or, the secondary structure of messenger RNA from a given stage of development may be specific and therefore recognized by a unique initiation factor.

Aminoacyl Transfer Ribonucleic Acid Synthetases from Cell-Free Extract of Plasmodium berghei

Aminoacyl transfer ribonucleic acid synthetases for leucine tyrosine, histidine, valine, proline, threonine, and lysine were obtainnned from cell-free extract of Plasmodium berghei. The leucyl-tRNA synthetase cane charge tRNA from liver or Escherichia coli with leucine-c14, liver tRNA being a better substrate. The amount of aminoacylation increses linerly with respect to the quantity of tRNA added from either source and is dependent on the amount of enzyme added. The rate of aminoacylation is constant for 10 minutes and then decreases. It is enhanced by polyvinylsulfate. One-tenth millimoler pyrimethamine, hydroxystilbamidine, quinacrine, and acriflavine inhibited the formation of C14-valyl-tRNA. Species specificity between tRNA and its charging enzyme with respect to the recognition site is discussed.

PROTEIN SYNTHESIS IN INSECTS

Publisher Summary This chapter provides an overview of protein synthesis in insects. The generality of the triplet code throughout the living world indicates the analogy between mechanisms for protein synthesis in various groups of organisms. Although a complete overlap in the mechanism of protein synthesis is evident for prokaryotic and eukaryotic organisms, there are some differences between the two large groups. From available data, it is apparent that the mechanism is the same or a variant of the same mechanism. The information for the amino-acid sequence of a particular protein is specified by a sequence of nucleotides in the DNA. Therefore, the process of protein synthesis begins with the transcription of DNA into intermediate RNA. This information is transferred in the living world through the triplet code. The site of protein synthesis is the cytoplasmic ribosomes to which mRNA becomes attached. Therefore, mRNA has to be transported from the site of synthesis, that is, the nucleus, to the cytoplasm. To elucidate the reactions leading to protein synthesis and to delineate the steps in the translational process, a cell-free system for protein synthesis was developed. The differentiated cell is recognized by its proteins, and the process of cell differentiation is linked to the mechanism that directs specific protein synthesis rather than to the ultimate appearance of protein molecules.