Molecular characterization of the mouse ribosomal protein S24 multigene family: a uniquely expressed intron-containing gene with cell-specific expression of three alternatively spliced mRNAs.

A family of 16 genes encoding the mouse ribosomal protein S24 was identified, and four members from this family were cloned. A single expressed intron-containing S24 gene (termed mrpS24) and one pseudogene (mrpS24p) were completely sequenced and characterized. The mrpS24 gene has seven exons and six introns spanning over 5.1 x 10(3) nucleotides (nt). The cap site of S24 was mapped to a G residue four nt upstream of a polypyrimidine tract and 15 nt downstream of a TATA-like (TATGA) element. The 5' region (-325 to +33) of the mrpS24 gene has a functional promoter that was able to express the fused chloramphenicol acetyltransferase (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), and the deduced amino acid sequence is missing a C-terminal lysine residue encoded by the other cDNA (S24b). The pseudogene mrpS24p is flanked by an 11-bp direct repeat, and its sequence is almost identical to the 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 demonstrated the existence of a third form (S24c) that similarly lacks both of the mini-exons, and suggested that different species of S24 mRNA might arise from alternative splicing of the mini-exons V and VI. Northern blot analysis showed that S24 expression is down- and up-regulated during adipocyte differentiation and in cellular transformation, respectively. RNase protection assays and RT-PCR experiments suggested that these cell-specific changes of S24 mRNA levels are mainly due to fluctuations in S24c mRNA level. Our results provide the first indication that a ribosomal protein gene is regulated by alternative usage of two mini-exons in a cell-specific manner.