Poly tract function

Genomic RNA. The viral genome is a positive-stranded RNA of approximately 11,700 nucleotides and has the structural features of messenger RNA (ie, mRNA, a 5 methylated cap [m7GpppA] and a poly-A tract at the 3 end).77 As a complete and functional mRNA, genomic RNA purified from virions is fully infectious when artificially introduced (ie, transfected) into susceptible cells. Similarly, RNA transcribed from a full-length complementary DNA (cDNA) clone of an alphavirus is also infectious,... 

While the poly (A) sequences do seem to be involved in the transport of mRNA s from the nucleus, this does not seem to be the sole function of the poly (A) tract for example, adenovirus DNA appears to lack a DNA sequence complementary to poly (A) but replicates in the nucleus of the mammalian cell and appears to have a poly (A) tract added to the viral mRNA by host-cell mechanisms for transport of the adenovirus mRNA to the cytoplasm (Philipson et ah, 1971). As with cellular messages, cordycepin blocks both the labeling of the poly (A) tracts and the appearance of adenovirus-specific RNA in the cytoplasm of infected cells (Philipson et ah, 1971). In contrast, vaccinia virus replicates exclusively in the cytoplasm of cells it infects and still contains poly(A) sequences (Kates, 1970). Since no role in transport is involved here, it suggests that some mRNAs may require a poly (A) sequence for proper translation. Further, not all mammalian mRNAs contain poly (A) and still are transported to the cytoplasm for translation. Specifically, the 9 S histone message isolated by Adesnik and Darnell (1972) from HeLa cells lacks any detectable poly (A) sequence of any significant length. These workers have also shown that the exit time of the histone mRNA molecule from the nucleus is shorter than that of other messenger RNA s. 

It was suggested that poly(c) tracts mi t play a role in the replication of those viruses which contain it (28, 29) In particular, they speculated that such a poly(c) tract mi t function as a replicase-binding site, or a part of one, on the basis of two observations. First, Rosenberg at al. (39) reported that a partially purified EMC viral replicase preparation will synthesise poly(G) from a poly(c) template, but will not copy other synthetic primers. Second, the replicase of bacteriophage which has subunits of similar size to those of the EMCV replicase (39) can also specifically use poly(C) as a template for poly(G) synthesis. However, the significance of this specificity is unclear, since Q3... 

It is interesting that the presence or absence of poly(c) coincides with the classification of enteroviruses, cardioviruses, rhinoviruses and PM) viruses as separate genera, proposed in (4). Frisby (l 5) has suggested that this implies a non-involvement of the poly(c) tract in a universal function such as replication, and that it might have some specific role in assembly, as a nucleation site, interacting with the capsid proteins of those viruses which contain it. The capacity of poly(C) to form double-stranded helices at pH 5 7 or below mi t contribute to the known instability of cardioviruses and FM) viruses at acid pH, which has been employed as a criterion in the classification of the picornavirus genera (4) ... 

The function of the poly(A) tracts which occur in cellular mENAs is still not clearly established. Possible roles have been suggested in (a) processing of precursors to mENAs (b) transport of mRNAs from the nucleus to the cytoplasm (c) efficient translation of mENAs (d) protection of mENAs against hydrolysis by exonucleases. 

We have very little idea about the stoichiometry of the two major polysomal mRNP proteins, or their location on the mRNA, apart from the fact that the 8,000 dalton protein seems to be associated with the poly A tract at the 5 end (59) The function of the proteins is even more obscure. In cell-free translation assays the activity of polysomal mRNP is no greater than that of deproteinised mRNA (20). This result is open to the qualification that in crude cell-free systems the added deproteinised mRNA mi t pick up proteins from the cell-free extract and thereby be effectively converted into polysomal mRNP particles, but since the same result is obtained in highly fractionated systems (11) this reservation can probably be discounted. The major polysomal mRNP proteins are distinct from all seven recognised initiation factors (ll), and initiation complex formation is found to require the same set of seven factors regardless of whether polysomal mRNP or deproteinised mRNA is used (11). In short, there is no evidence that these proteins play a role in mRNA translation. 

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