CLONING, EXPRESSION, AND CHROMOSOMAL LOCALIZATION OF THE RAT MITOCHONDRIAL CAPSULE SELENOPROTEIN GENE (MCS), THE READING FRAME DOES NOT CONTAIN POTENTIAL UGA SELENOCYSTEINE CODONS

The mitochondrial capsule selenoprotein (MCS) is a selenium-containing polypeptide. It is one of three proteins that are important for the m aintenance and stabilization of the crescent structure of the sperm mi tochondria. In this paper, we report the isolation and characterizatio n of the rat MCS cDNA and gene. The cDNA contains a reading frame for a 145-amino-acid protein and it lacks the UGA codons, which have been found in the reading frame of the mouse MCS cDNA and have been presume d to encode the selenocysteine in the amino terminal of the deduced mo use amino acid sequence. The deduced amino acid sequence of the rat an d mouse MCS shows a high level of homology (79%). The rat MCS gene con tains two exons; the intron sequence interrupts the 5' untranslated se quence at the same position as in the mouse MCS gene. The transcriptio n start site is located 184 bp upstream of the translation start site. Alignment of the 5'-flanking regions of the mouse and rat genes revea ls that the first 400 nucleotides upstream of the transcription start site exhibit an overall sequence similarity of 73%. This conserved reg ion contains no TATA or CAAT box motifs. Northern blot analysis indica tes that the MCS mRNA is detectable only in the testis after day 30 of postnatal development. Moreover, in situ hybridization revealed that the rat MCS gene is mainly expressed in round spermatids. From the ana lysis of mouse-rat cell hybrids that segregate rat chromosomes, the MC S gene was assigned to rat chromosome 2.