Polysome formation

From the above discussion, it appears that zinc may have its primary effect on zinc-dependent enzymes that regulate the biosynthesis and catabolic rate of RNA and DNA. In addition, zinc may also play a role in the maintenance of polynucleotide conformation. Sandstead et al. (99) observed abnormal polysome profiles in the liver of zinc-deficient rats and mice. Acute administration of zinc appeared to stimulate polysome formation both in vivo and in vitro. This finding is supported by the data of Femandez-Madrid, Prasad, and Oberleas (42), who noted a decrease in the polyribosome content of zinc-deficient connective tissue from rats and a concomitant increase in inactive monosomes. 

Many other schemes may also be drawn. To determine which scheme is correct, it will be necessary to answer the following questions l.What is the exact localization of free cytoplasmic informosomes in the cell 2. Do nuclear D-RNP, cytoplasmic informo-somes, and polysome-bound mRNP have the same nature In particular do they contain the same protein moiety or not 3. What is the nature of intermediate complexes in the process of polysome formation and where are the polysomes formed ... 

A similar situation has recently been reported for diabetic man . While a two to threefold Increase of several ketogenic amino acids was found to occur in the plasma of ketoacidotic diabetic patients, a 25-40 percent reduction in the levels of glucogenic amino acids was observed. The mechanisms by which insulin stimulates protein synthesis and reduces amino acid mobilization are poorly understood. It is possible that a fundamental action of insulin on protein metabolism may be the induction of a "translation factor" which in turn allows for polysome formation . It is still not clear, however, to what extent an insulin effect at the cell membrane or an insulin effect on amino acid flux across the cell is necessary for observed effects at the nucleic acid level . ... 

Surprisingly, this treatment (excision of the apex) led to polysome formation which was greater 1 h after excision than 10 h after auxin treatment of the intact plant [8]. In efforts to circumvent this wound-evoked polysome formation, we excised the tissue at a point 5 cm below the apex and were again surprised. Polysome formation in the apical 1 cm was almost as massive as in tissue wounded at the 1 cm point. In fact, polysome formation took place in the apical 1 cm within 15 min of inflicting a wound at a point 20 cm distant. This led to our realization that there was a rapidly-generated (and bidirectionally-transmitted) wound signal that could very rapidly elicit polysome formation in distant tissue [8, 18]. 

We have not yet been able to prove (or disprove) conclusively that APs are the signals which evoke polysome formation. Attempts to mimic wounding electrically show that stimulations of sufficient voltage to pass the cuticle do, in fact, elicit an AP and evoke the formation of polysomes. Unfortunately, such treatments also cause severe damage and even death of the cells in the region stimulated (data not shown). Thus AP generation and polysome formation may be the result of wounding, and not of electrical stimulation directly. 

As shown in Fig. 1, exposure of a 5-mm region of the epicotyl to UV 20 cm distant from the apex results in polysome formation in the apical 2 cm which is more massive than that induced by wounding. Others have also shown that UV evokes APs [5]. Furthermore, cold treatment of the middle of a stem (which we had hoped would prevent transmission of APs) did not prevent polysome formation at one end when the tissue was wounded at the other end. In fact cold treatment alone elicited polysome formation (not shown). Again, others have shown that cold treatment can elicit (not inhibit as we had originally presumed) APs [5]. 

Fig. 1. Polysome formation in aged peas in response to U V irradiation and wounding. Aged pea plants were either left untreated control), wounded at the tip close wound), or at 20 cm distant wound), or irradiated with UV for 15 min at the tip close VV)or

Effects of Endogenous Wound-Induced Gaseous Hormones and Ethylene Inhibitors on Polysome Formation... 

Incubation of unwounded tissue in a closed container in the presence of several pieces of wounded tissue does not cause polysome formation, whereas treatment with the gaseous inhibitor of ethylene action, norbornadiene does (not shown). This... 

Table 2. Effect of ethylene on polysome formation in aged, excised pea epicotyls ...

GA enhances polysome formation and protein synthesis Disputed... 

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