Structure of the bovine transducin gamma subunit gene and analysis of promoter function in transgenic mice

Transducin, the major photoreceptor guanine nucleotide-binding protein (G protein), is composed of three polypeptides: alpha, beta and gamma subunits. The transducin gamma subunit (T gamma) is expressed preferentially in photoreceptors. To study the control mechanisms for photoreceptor-specific expression of the T gamma gene, clones of the bovine T gamma gene were isolated from a bacteriophage genomic library, and the structure of the gene, including a portion of its 5'-flanking region, was characterized. The gene consists of three exons and two introns. The first intron is 91 base pairs (bp) long and is located in the region corresponding to the 5'-untranslated sequence of the T gamma mRNA. The second intron is 5.3 kilobases (kb) long and splits the protein-coding region centrally. A bovine Alu-type repetitive sequence and putative Ret-1 and AP-1 binding site sequences are located in the 5'-flanking region. To investigate promoter function, 1.4 kb of DNA from the 5'-flanking region was joined to the prokaryotic chloramphenicol acetyltransferase (CAT) gene, and the chimeric bovine T gamma gene was used to generate a line of transgenic mice. CAT activity was readily detected in the retinas of the transgenic mice, but was absent in brain, heart, kidney, liver, lung, spleen and other tissues. These results suggest that the 1.4 kb 5'-flanking region of the bovine T gamma gene contains conserved sequence elements that direct tissue-specific expression. Human T gamma cDNA clones were characterized, and a short homologous region of the human T gamma gene promotor was obtained by polymerase chain reaction (PCR) amplification for comparison with the bovine promoter.