AbstractIn vitrocultures of human retinal pigment epithelial (RPE) cells were used to study the regulation of basic fibroblast growth factor (bFGF) gene expression. Four transcripts of 7.0, 3.7, 2.2, and 1.2kB are produced from the bFGF gene. Increasing cell density has a profound effect on the expression of the 7.0 kB transcript relative to the 3.7 kB transcript. Here, evidence is presented suggesting that posttranscriptional processing events are responsible for differential expression of the 7.0 and 3.7kB bFGF transcripts as a function of cell density. Primer extension analysis demonstrates that these two transcripts originate from a single transcription initiation site. Determination of the half-lives of the 7.0 and 3.7kB transcripts at confluent cell density did not explain the relative expression of these mRNAs. These differences may arise from the use of alternative polyadenylation sites in the 3' untranslated region (UTR) as a function of cell density. Polysomal analysis indicates no selective translation of any of the four bFGF transcripts in RPE cells.