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We have constructed deletion and point mutations within the Simian virus 40 (SV40) early promoter region which contains two tandemly repeated 21 bp sequences and a related 22 bp sequence (the "upstream" 21 bp repeat region). After transfection into permissive CV-1 cells and non-permissive mouse 3T3-4E cells, the effect of the mutations on early gene expression was studied by measuring T-antigen production, using indirect immunofluoresence. Our results demonstrate that the 21 bp repeat region, and in particular the six GC-rich motifs 5'-CCGCCC-3' which are repeated in this region constitute an important element of the SV40 early promoter. Surprisingly, we found that the requirement for the 21 bp repeat region for early gene expression was partially fulfilled even when it was in the inverted orientation.

Using quantitative S1 nuclease analysis and recombinants which contain the SV40 early promoter region linked to the rabbit beta-globin gene coding sequence, we have studied the effect of deletion, inversion and point mutations, located within the 21 bp repeat region, on initiation of transcription from the early-early and late-early startsites in the absence of T-antigen. Our data establish unequivocally that the six GC-rich repeats present in the 21 bp repeat region are essential elements of both the early-early and late-early promoters and that they are not redundant, since mutations, which affect only the GC-rich repeat most proximal to the TATA box, decrease drastically the activity of the early-early promoter, but increase that of the late-early promoter. On the other hand, the four GC-rich repeats most proximal to the 72 bp repeat are common elements of the two overlapping early-early and late-early promoters. Our results, which confirm that the early-early promoter is stronger than the late-early one, also support our previous suggestion that they are in competition for the transcriptional machinery. The general organization of the SV40 early promoter region is discussed.

We have analysed by S1 nuclease mapping the in vivo termination sites of transcription of the caa-cal operon and cai gene. The termination region for caa mRNA (T1A terminator) features characteristics of a rho-independent terminator. This terminator is a convergent transcription terminator, its complementary secondary structure being present at the 3'-end of cai mRNA. The caa-cal mRNA terminator (T2A terminator) has a stable potential secondary structure and shows homology with rho-dependent terminators. In vitro transcription of caa-cal operon demonstrated that the two terminators T1A and T2A are efficient. The 3'-ends of the mRNAs which end at T1A and T2A were analysed by S1 mapping with total RNA purified from a mutant strain deficient in exoribonuclease activities, in particular RNase II. The results suggest that the potential secondary structures of T1A and T2A are sufficiently stable to prevent 3'-end degradation by RNase II. On the other hand, the T2A terminator should be efficient enough to stop transcription through the downstream DNA region involved in pColA replication.