NATURALLY-OCCURRING ANTISENSE TRANSCRIPTS ARE PRESENT IN CHICK-EMBRYOCHONDROCYTES SIMULTANEOUSLY WITH THE DOWN-REGULATION OF THE ALPHA-1(I) COLLAGEN GENE

It has previously been shown that very low steady state levels of alph a 1(I) collagen mRNA are present in chick embryo sternal chondrocytes (Askew, G. R., Wang, S., and Lukens, L. N. (1991) J. Biol. Chem. 266, 16834-16841), yet nuclear run-on experiments with double-stranded cDNA probes indicated a high transcription rate at this locus. These findi ngs were investigated in this study using single-stranded probes, wher e nuclear run on experiments showed that antisense transcription of th e alpha 1(I) collagen gene was occurring in chondrocytes, while sense strand transcription was down-regulated. Treatment of these chondrocyt es with 5-bromo-2'-deoxyuridine (BrdU), which causes the cells to rese mble their mesenchymal precursors, resulted in an antiparallel situati on, where antisense transcription was lost, and instead, sense strand transcription was acquired, suggesting that the reverse switch from se nse to antisense transcription occurs during chondrogenesis. Very larg e (> 10 kilobases) and heterogeneous antisense transcripts of moderate stability were shown to span both ends of the gene in chondrocytes, w hile their absence was shown in BrdU-treated chondrocytes, chick embry o fibroblasts, and a variety of other tissues. The function of these a ntisense transcripts is so far unknown, but their unusual chondrocyte- specific appearance, concurrent with little or no sense strand transcr iption, suggests a possible functional role in the down-regulation of the alpha 1(I) collagen gene.