Detection of 28S RNA with the FGF-2 cDNA at high stringency through related G/C-rich sequences

Fibroblast growth factor-2 (FGF-2) or basic FGF is a multifunctional protei n that, through interaction with specific cell surface receptors, plays imp ortant roles in the growth and development of tissues and organs. Thus, con siderable attention has focused on the control of FGF-2 gene expression, in cluding assessments of RNA levels through blotting and the use of radiolabe led FGF-2 cDNA probes. Multiple transcripts of different sizes have been re ported for FGF-2 by this approach, however, more recent evidence indicates that at least one of these RNAs of about 1.5 kb, is not an authentic FGF-2 transcript. A major band of 4.7 kb and a minor band of 6.1 kb were detected in total rat glial tumor cell RNA, using the 'intact' rat ovarian FGF-2 cD NA as a probe at high stringency. This cDNA contains both coding and 5'-unt ranslated sequences. Although the 6.1 kb transcript levels were increased i n RNA enriched for polyadenylated species, the levels of the 4.7 kb band we re decreased and also shared a mobility with 28S RNA. A truncated FGF-2 cDN A probe, containing coding but not 5'-untranslated sequences, detected the 6.1 kb transcript but failed to see the 4.7 kb band. The domain responsible for detecting the 4.7 kb band was localized to a G/C-rich region containin g 5'-untranslated sequences, by using different fragments of the rat FGF-2 gene, including coding and upstream flanking DNA, as probes. The degree of similarity between sequences of this G/C-rich region of the FGF-2 gene and 28S RNA from rat, human and mouse was sufficient to predict strong cross hy bridization. This was confirmed by the detection of a 4.7 kb band in mouse heart RNA with the 'intact' but not truncated rat FGF-2 cDNA probes; a 6.1 kb mouse FGF-2 transcript was detected with both probes. These data indicat e that the 4.7 kb RNA detected is not a bona fide FGF-2 transcript, and mos t likely represents cross hybridization with abundant 28S RNA through G/C-r ich non-coding sequences present in the 'intact' rat FGF-2 cDNA. However, s equence comparisons suggest that this result may be the case for other spec ies and might not be restricted to the rat FGF-2 cDNA.