MOLECULAR CHARACTERIZATION OF THE MOUSE RIBOSOMAL-PROTEIN S24 MULTIGENE FAMILY - A UNIQUELY EXPRESSED INTRON-CONTAINING GENE WITH CELL-SPECIFIC EXPRESSION OF 3 ALTERNATIVELY SPLICED MESSENGER-RNAS

A family of 16 genes encoding the mouse ribosomal protein S24 was iden tified, and four members from this family were cloned. A single expres sed intron-containing S24 gene (termed mrpS24) and one pseudogene (mrp S24p) were completely sequenced and characterized. The mrpS24 gene has seven exons and six introns spanning over 5.1x10(3) nucleotides (nt). The cap site of S24 was mapped to a G residue four nt upstream of a p olypyrimidine tract and 15 nt downstream of a TATA-like (TATGA) elemen t. The 5' region (-325 to +33) of the mrpS24 gene has a functional pro moter that was able to express the fused chloramphenicol acetyltransfe rase (CAT) reporter gene. Two different forms of mouse S24 cDNA clones were previously isolated. Sequence analysis showed that one of these cDNA clones (termed S24a) lacks the entire exon V sequence (18 nt), an d the deduced amino acid sequence is missing a C-terminal lysine resid ue encoded by the other cDNA (S24b). The pseudogene mrpS24p is flanked by an 11-bp direct repeat, and its sequence is almost identical to th e S24 cDNA sequence, but it lacks two mini-exons, V and VI (20 nt), as in the cases of the human and rat S24 cDNAs. RT-PCR experiments demon strated the existence of a third form (S24c) that similarly lacks both of the mini-exons, and suggested that different species of S24 mRNA m ight arise from alternative splicing of the mini-exons V and VI. North ern blot analysis showed that S24 expression is down- and up-regulated during adipocyte differentiation and in cellular transformation, resp ectively. RNase protection assays and RT-PCR experiments suggested tha t these cell-specific changes of S24 mRNA levels are mainly due to flu ctuations in S24c mRNA level. Our results provide the first indication that a ribosomal protein gene is regulated by alternative usage of tw o mini-exons in a cell-specific manner.