Early capping of transcripts from the adenovirus major late transcription unit

The various events which result in modification of a primary RNA transcipt may have a role in choosing transcripts which are to be processed into mRNA. If there is a stepwise and interdependent nature to each of the modifications then it is important to place the various steps in temporal order. We show here that the formation of a capped 5' terminus seems to be a very early event for adenovirus type 2 (Ad-2) nuclear RNA that is initiated at the major late Ad-2 promoter. There is an equally rapid entry of 3H-adenosine into the cap and the first dozen or so adenylate residues in the RNA chain. Taken together with evidence on general mRNA metabolism in Chinese hamster cells, it seems unlikely that capping has any differential role in successful RNA processing rather is an automatic event for all RNA polymerase II products.

Regulation of early adenovirus transcription: a protein product of early region 2 specifically represses region 4 transcription

An aspect of the regulation of early adenovirus gene expression has been studied by measuring the rates of transcription of several viral transcription units during infection by wild-type (WT) adenovirus type 5 and a temperature-sensitive mutant, H5ts125. It has previously been shown that two transcription units (regions 2 and 4) are subject to negative control. During the course of early infection, transcription of regions 2 and 4 increased to a maximal rate and then declined. The decline from each of these transcription units appeared to be a specific shutoff of transcription, because the transcription of another early transcription unit (region 1A) remained constant during early infection and, in the absence of protein synthesis, the apparent shutoff of region 2 and region 4 transcription did not occur. At the nonpermissive temperature for the mutant, the kinetics of transcription of early region 2 were identical to the kinetics of transcription of WT. Thus, the repression of transcription of region 2 occurred in H5ts125-infected cells as well as in WT-infected cells. Region 4 transcription, however, was not repressed during H5ts125 infection at the nonpermissive temperature. After a maximal rate of transcription was reached, this rate was maintained throughout the course of the experiment. Furthermore, the repression of region 4 transcription appears to be the result of a block of initiation of transcription rather than the result of an inducement of premature termination of transcription. Therefore, it can be concluded that a product of the adenovirus region 2 gene, which is the site of the mutation for H5ts125, is responsible for the specific shutoff of region 4 transcription.

Regulation of the primary expression of the early adenovirus transcription units

The time course of appearance of transcriptional activity from five early adenovirus type 2 transcription units has been determined. RNA complementary to region 1A (1-4.4 map units), the first region to be transcribed, was detectable at 45 min after infection; a maximal rate of RNA synthesis was reached at 3 h after infection and was maintained thereafter for at least 6 h. RNA from region 2 (75-56 map units), which encodes the mRNA for the 72,000-dalton DNA-binding protein, was the last to be synthesized; transcription commenced at about 2 h postinfection, reached a maximum at 7 h, and then declined. Transcription of regions 3 (76-86 map units) and 4 (99-91 map units) reached a maximal value at 3 h postinfection. The rates of RNA synthesis from these regions then declined over the next 6 h. The decline of transcription from regions 2 and 4 appeared to be a specific repression of these transcription units. The repression did not occur in the absence of protein synthesis, suggesting that a viral protein might be involved. Transcription of all early regions was initiated and continued for at least 2 to 3 h in cells that were treated with cycloheximide or emetine before and during infection, suggesting that at least the initiation of RNA synthesis from the five early adenovirus type 2 transcription units does not depend on the formation of a viral protein. Moreover, mRNA was formed in the absence of protein synthesis that hybridized to DNA fragments representing each of the five early transcription units. The increase in mRNA accumulation in the presence of cycloheximide (or emetine) does not appear to be due to increased RNA synthesis; thus, either increased mRNA stability or increased efficiency of nuclear RNA processing must occur.

Steps in the processing of Ad2 mRNA: poly(A)+ nuclear sequences are conserved and poly(A) addition precedes splicing

The conservation of nuclear Ad2 sequences during nucleocytoplasmic transport has been estimated from the accumulation of 3H-uridine in nuclear and cytoplasmic Ad2-specific RNA from the major late transcription unit. From 10-28% is conserved of the total Ad2 nuclear RNA synthesized from each of five regions of the genome that specify groups of 3' co-terminal mRNAs. The sum of the conservation of all the regions was equivalent to 100%, signifying the conservation of at least a part of each transcript or all of about one fifth to one sixth of the transcripts. The conservation of poly(A)+ Ad2 nuclear RNA is about 4 times greater than of total Ad2 nuclear RNA, approaching 100% conservation of poly(A)+ nuclear sequences. Since each mRNA contains three "spliced" sequences that are probably encoded only once per transcript, these data on conservation of the Ad2 sequences suggest that each transcriptional event from the 16-99 transcription unit gives rise to one of a possible 13-14 mRNA molecules with destruction of the remainder of the transcribed RNA. The portion which is conserved resides next to the region to which which poly(A) is added. Three models for the choice of poly(A) sites were considered: termination at the poly(A) site, cleavage shortly after synthesis of one of the sites before transcription was complete, and cleavage after completion of transcription. The first model was ruled out by the demonstration of equimolar synthesis over the 16-99 region. The second model is strongly supported because 3H-uridine label appears equally rapidly in the time range 2-10 min in each of the five 3' poly(A) addition sites, whereas chain completion before cleavage would lead to a faster appearance of label in the most promoter-distal site. Furthermore, briefly labeled RNA molecules extending from 16 to each of several poly(A) addition sites were the first poly(A)- terminated 3H-uridine-labeled molecules detected, demonstrating that poly(A) addition precedes splicing. The choice of which mRNA emerges from each transcriptional event would appear to depend upon first choosing one of five 3' mRNA ends followed by a 5' splicing event.

Groups of adenovirus type 2 mRNA's derived from a large primary transcript: probable nuclear origin and possible common 3' ends

Late in adenovirus type 2 infection, a number of mRNA's apparently arise by processing a large nuclear transcript that represents the right-hand 85% of the genome (summarized in Evans et al., Cell 12:733-739, 1977). Hybridization of labeled late mRNA to a series of DNA restriction fragments representing the right-hand 70% of the genome demonstrates at least 12 discrete mRNA's that appear to fall into five groups, each possibly containing a common 3' terminus. The processing necessary to generate these mRNA's apparently occurs in the nucleus. All the mRNA's appear to contain a sequence of approximately 100 nucleotides complementary to a fragment with coordinates 25.5-27.9. This fragment contains one of the regions found by Berget et al. (Proc. Natl. Sci. U.S.A. 74:3171-3175, 1977), Chow et al. (Cell 12:1-18, 1977), and Klessig (Cell 12:9-22, 1977) to the "spliced" onto the 5' end of late adenovirus type 2 mRNA's. Because the sequences to be spliced exist only once per large transcript, any of the mRNA-specific regions might only be preserved from a small fraction of the transcripts. Measurement of the transport efficiency of regions of the large nuclear transcript, if fact, shows that only 15 to 25% of any particular region is transported to the cytoplasm. The overall conclusion of these experiments is that the large late nuclear transcript can be processed in the nucleus to yield any one of many (approximately 12) mRNA's; the unused portions of the primary transcript then accumulate in the nucleus or are destroyed.