Cycloleucine, a competitive inhibitor of methionine transferase was used to 
generate in vivo partially methylated mRNA in SV40-infected BSC-1 cells. 
Cycloleucine at 0.5 mg/ml causes more than a 30% decrease in internal m6As of 
late SV40 mRNA with only minor effect on the dimethyladenosine of the 5' caps 
m7GpppmAm. After treatment with 2 and 5 mg/ml of cycloleucine, internal m6As 
were reduced by 10- and 100-fold, respectively. The inhibition of BSC-1 mRNA 
methylations paralleled that observed for late SV40 mRNAs. In cells exposed to 2 
mg/ml cycloleucine production of late SV40 mRNA was inhibited by 80% whereas the 
amount of SV40 nuclear RNA was only slightly reduced. Size fractionation of SV40 
nuclear RNA from cycloleucine-treated cells revealed a loss of SV40 19 S RNA 
with a corresponding increase of fragmented RNA sedimenting between 11 to 5 S, 
so that the total amount of SV40 RNA in the nucleus was almost unchanged. 
Analysis of viral transcription complexes from cells treated with cycloleucine 
indicated that SV40 transcription was not affected by cycloleucine. 
SV40-transformed cells, in contrast to BSC-1 cells, were able to process and 
transport undermethylated RNA. When transformed cells were treated with 2 mg/ml 
cycloleucine no changes in quantities or size of cytoplasmic and nuclear RNA 
were detected. The data argues for a role of internal m6A moieties in modulating 
the processing-linked transport of mRNA from the nucleus to the cytoplasm of 
nontransformed cells. Transformed cells may escape these controls due to 
structural alterations in their perinuclear regions.