The Gnb5 gene is a novel beta-transducin homolog transcribed from a divergent promoter located immediately upstream of the Syrian hamster p53 P1 promoter.

Regulatory regions controlling p53 gene transcription in Syrian hamster embryo cells were characterized by use of chloramphenicol acetyl-transferase (CAT) constructs encompassing various subfragments of its 5'-flanking sequences. This analysis identified a 961 bp PstI-SacI (PS) fragment upstream from the p53 P1 promoter, which exhibited promoter activity only in the reverse orientation relative to the p53 gene. Northern hybridizations of mRNA from hamster embryo cells with genomic probes containing the PS fragment detected a 2.1-kb transcript expressed at much lower levels than the p53 mRNA. Steady-state levels of the 2.1-kb mRNA were threefold higher in actively growing cells than in cells from confluent cultures. Library screenings with PS-containing probes resulted in the isolation from exponentially growing cells of a cDNA, the nucleotide sequence of which showed no significant homology to genes previously described. This novel gene, named Gnb5, for guanine nucleotide-binding protein, beta 5, codes for a protein of 538 amino acids with a highly acidic amino terminus containing a proline-rich domain, followed by a neutral domain with five repeat units of the beta-transducin (WD-40) motif. The homology with beta subunits of G proteins and with other WD-40 repeat-containing proteins was restricted to the repeats. The Gnb5 gene is well conserved in rodents and primates, as the hamster Gnb5 cDNA recognized, under high stringency conditions, the human and mouse counterparts in Southern and Northern hybridizations. Expression of Gnb5 in adult tissues was detected preferentially in testes, in both hamsters and humans.