The primary transcript from adenovirus 2 early region 1B (E1B) is processed by differential RNA splicing into two overlapping mRNAs, 13S and 22S. The 22S
mRNA is the major E1B
mRNA during the early phase of
infection, whereas the 13S
mRNA predominates during the late phase. In previous work, it has been shown that this shift in proportions of the E1B mRNAs is influenced by increased cytoplasmic stability of the 13S
mRNA at late times in
infection. Two observations presented here demonstrate that the increase in proportion of the 13S
mRNA at late times is also regulated by a change in the specificity of RNA splicing. First, the relative concentrations of the 13S to 22S nuclear RNAs were not constant throughout
infection but increased at late times. Secondly, studies with the mutant, adenovirus 2 pm2250 , provided evidence that there was an increased propensity to utilize a 5' splice in the region of the 13S
5' splice site at late times in
infection. Adenovirus 2 pm2250 has a G----C transversion in the first base of E1B 13S
mRNA intron preventing splicing of the 13S
mRNA but not of the 22S
mRNA. During the early phase of a pm2250
infection, the E1B primary transcripts were processed into the 22S
mRNA only. However, during the late phase, when the 13S
mRNA normally predominates, E1B primary transcripts were also processed by RNA splicing at two formerly unused or cryptic
5' splice sites. Both
cryptic splice sites were located much closer to the disrupted 13S
5' splice site than to the 22S
5' splice site. Thus, the temporal increase in proportion of the 13S
mRNA to the 22S
mRNA is regulated by two processes, an increase in cytoplasmic stability of the 13S
mRNA and an increased propensity to utilize the 13S
5' splice site during the late phase of
infection. Adenovirus 2 pm2250 was not defective for productive
infection of HeLa cells or for transformation of rat cells.