The Cistron

While regulatory roles for repetitive transcripts have been suggested, at least some mRNA species originate from repeated DNA base sequences. Bishop et al. (1972) utilized the vast DNA excess hybridization procedure to demonstrate that the cistrons for the alpha and beta chains of hemoglobin are present at a reiteration frequency of about five each. A much higher reiteration frequency is reported for the cistrons coding for histone mRNA (Kedes and Birnstiel, 1971). However, the majority of polysomal mRNA molecules seem to be derived from nonrepeated sequences (Greenberg and Perry, 1971 Firtel etal., 1972). 

At various times during embryonic development there seem to be very distinct requirements for rapid synthesis of rRNA, 5 S RNA, and tRNA, and the rates of synthesis in such instances may be very much higher than the values reported by Quincey and Wilson (1969). However, gene products which are supposedly derived from repetitive DNA base sequences are reached by an operational definition. It is clear that heterogeneity of the cistrons for tRNA exists and rates of synthesis may affect only some, and not all, of these cistrons. Similarly, heterogeneity of the rRNA cistrons has been suggested (Moore and McCarthy, 1968) and sequence studies clearly show that more than one type of 5 S RNA exists in Xenopus (Ford and Southern, 1973). 

The observations on the histidine operons described above clearly show that all the enzymes with amino acid sequences determined by the histidine operon are not synthesized in equimolecular amounts. But the rate at which each of the proteins is synthesized is determined by the position of the cistron within the operon. The amount of protein synthesized decreases the further the cistron is from the operator gene. Furthermore, mutations that interfere with the biosynthesis of one enzyme of the histidine pathway lead to a reduction of the rate of synthesis of all the enzymes with structures dictated by cistrons distal to the mutated cistron. 

The anticodon may either correspond to a triplet sequence normally involved in dictating the position of a given amino acid, or it is a modulating triplet, which facilitates the disconnection of the ribosome from the messenger. Thus, the chances for a ribosome to fall off its mRNA increase as the distance between the reading point and the operator increases. The position of the cistron within the polycistronic operon and the existence of modulating triplets could well explain the modulation of protein synthesis, or rate at which different proteins are synthesized. When the polarity is modified as a result of a mutation, it is assumed that a coding triplet is replaced by a modulation triplet. 

While the number of transcribed cistrons is unknown, this number is apparently small. Also, there is an unknown amount of aggregation between the products of the cistrons involved. 

Since virus RNA of poliovirus, despite its polycistron character, does not undergo fragmentation when supplying information for synthesis of several proteins (early and late) or when functioning in the cell, but preserves its stability and activity for a long time, we can fully understand the desire to see this information in the linear order in which it is read and the reason underlying the temporal distribution of proteins synthesized. In this case it can be postulated that if, for example, the cistrons A, B, C, D, E, F,... 

The other variant (Fig. 11a) envisages that each separate cistron has its own specific point for attachment of ribosomes, so that the cistrons can carry out synthesis of individual proteins independently and at different speeds. In this case the partially loaded templates have the structure shown in Fig. 11a (top). Regulation and switching individual cistrons "on and "off in this case likewise can be independent. In the opinion of Rich and co-workers, intermediate cases are also possible. 

The scheme (Fig. 11a) thus provides a possible explanation of the synthesis of different virus proteins at different times and in different amounts (at different speeds), but it does require the existence of an as yet unknown mechanism of independent operation of individual cistrons in accordance with the program for virus development. In other words, it requires a programming mechanism, controlling protein synthesis by the polysome in the same way as a tune is produced from notes obtained by striking the keys of a piano. The scheme (Fig. 11b) resembles the simple repetition of a scale of successive sounds. In this case no notes, i.e.,programmed regulation of activity, are required it is assumed that control is by simple linear succession of the cistron-keys. 

Goldberger and Berberich (1965) carried out some interesting work to determine the time sequence of activity of the cistrons of the histidine operon based on the kinetics of repression and derepression of five (of the ten) enzymes of this operon from the beginning, middle, and end regions of the histidine operon of S. typhimurium. The specific activity of these enzymes was measured during a period soon before and after removal of histidine from the growth medium of the "leaky" mutant of this microorganism. 

Fig. 31. Genetic map oi Escherichia co/r (Taylor and Thoman, 1964). The map is graduated in intervals of 1 minute and has a total length equal to the time taken for transfer of the chromosome during conjugation (89 min). The symbols denote ability to synthesize threonine (thre), leucine (leu), proline (pro), lysine (lys), arginine (arg), biotin (biot), pyrimidines (pyr), etc. The capital letters for some polygenes (try, ara, etc.) denote the order of the cistrons within the operon.

So here we have, at the very least, strong evidence that the structural gene for a ribosomal component of the chloroplast is located in the nuclear genome (perhaps together, with at least one other which has been identified) and the cistrons for the ribosomal RNA are located in the chloroplast genome (probably together, with at least one gene for a chloroplast ribosomal protein [LC4]). How did this come about ... 

The cistron for globin would be selected, stabilized, and transferred to the cytoplasm, whereas the remaining RNA of this product would be degraded. At present there is little evidence on the basis of which one may choose between these possibilities. It is relevant to note, however, that treatment of avian erythrocytes with actinomycin D may result in inhibition of histone synthesis while the synthesis of hemoglobin is unimpaired (Freedman et al, 1966), a finding which would favor the first interpretation. 

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