A systematic survey of repetitive sequences abundantly expressed in rat tumors.

A cDNA library made from poly(A)+ RNA of a rat ascites hepatoma cell line, AH60C, was screened with respect to transcribed repetitive sequences by probing with AH60C genomic DNA. Thirty clones, comprising 1.2% of the library, were found to contain repetitive elements. The sequences of 12 clones among them were abundantly expressed in poly(A)+ RNA of AH60C, yet sparse in that of rat normal liver. This differential expression was also noted in case of other tumors and normal tissues tested. These 12 clones could be divided into two classes, depending on whether their sequences were expressed in cytoplasmic poly(A)+ RNA (class I) or not (class II). Rat Alu-like type II (B2) repeats and an unknown repetitive sequence were found in the clones of class I. Adjacent to each of them, we detected two non-repeated sequences, the expressions of which were also activated. The presence of another unknown repetitive sequence was observed in the clones of class II. It is noted that more than one-third of the transcribed repetitive sequences were involved in the predominant expression in tumor cells, whereas the majority of overall transcripts have been found to be common to both transformed cells and their normal counterparts.