Expression of adenovirus-2 early region 4: assignment of the early region 4 polypeptides to their respective mRNAs, using in vitro translation

Abstract

Adenovirus-2 early region 4 (E4; map positions 91.3 to 99.1) encodes six 5' and 3' coterminal, differently spliced mRNAs, which are 2.5, 2.1, 1.8, 1.5, 1.2, and 0.8 kilobases (kb) long. Hybridization selection with five cloned viral DNA fragments that hybridize with subsets of E4 mRNAs was used to purify these six mRNAs and a previously unreported 3.0-kb mRNA from virus-infected cells. E4 mRNAs which were purified by hybridization selection with cloned EcoRI fragment C (map positions 89.7 to 100) were also fractionated by size. The purified mRNAs were then translated in rabbit reticulocyte or wheat germ lysate systems. The full complement of E4 mRNAs specified as many as 16 different polypeptides, with molecular weights ranging from 24,000 (24K) to 10K. The most abundant E4 mRNA, which was 2.1 kb long, specified an 11K polypeptide. The 1.5-kb mRNA, which differed from the 2.1-kb mRNA only by deletion of a second intron from the 3' untranslated region, also specified an 11K polypeptide. The second most abundant mRNA, which was 1.8 kb long, and the 1.2-kb mRNA, which had an intron deleted from the 3' untranslated region, specified a 15K polypeptide. This polypeptide was labeled more intensely with [5,6-(3)H]leucine than with [35S]methionine. The 3.0- and 2.5-kb mRNAs specified four polypeptides (24K, 22K, 19K, and 17K). Translation of E4 mRNAs with a mean size of 0.8 kb, which accumulated preferentially in the presence of cycloheximide, yielded at least 10 polypeptides that migrated in polyacrylamide gels with apparent molecular weights ranging from 21,800 to 10,000. On the basis of translation in wheat germ lysates and the distribution of polypeptides encoded by size-fractionated mRNAs, we concluded that the 0.8-kb mRNA size class includes a heterogeneous mixture of mRNAs which are probably formed as the result of utilization of alternate splice acceptor and donor sites during removal of the second intron. Our polypeptide assignments for the 2.1-, 1.8-, 1.5-, and 1.2-kb mRNAs are compatible with locations of two open coding regions in the DNA sequence (Herisse et al., Nucleic Acids Res. 9:4023-4042, 1981). The relationship between the four polypeptides encoded by the 3.0- and 2.5-kb mRNAs and the two open coding regions is discussed. The production of multiple polypeptides from a heterogenous mixture of mRNAs in the 0.8-kb size class is compatible with two large open coding regions in that part of the sequence. Thus, nearly all of the potential coding information in the leftward strand of E4 is expressed in translatable form during infection. Moreover, alternate splicing of the 0.8-kb mRNA size class can produce multiple polypeptides with common amino-terminal and different carboxy-terminal amino acid sequences, which may have the same function but different specificities.