Alternative pre-mRNA splicing is often controlled by cell signals, for example, those activating the cAMP-dependent protein kinase (PKA) or the Ca 2؉ /calmodulin-dependent protein kinase IV (CaMKIV). We have shown that CaMKIV regulates alternative splicing through short CA repeats and hnRNP L. Here we use a splicing reporter that shows PKA/CaMKIV promotion of exon inclusion to select from exons containing random 13-nt sequences for RNA elements responsive to the kinases in cultured cells. This selection not only identified both PKA-and CaMKIV-responsive elements that are similar to the CaMKIVresponsive RNA element 1 (CaRRE1) or CA repeats, but also A-rich elements not previously known to respond to these kinases. Consistently, hnRNP L is identified as a factor binding the CA-rich elements. Analyses of the motifs in the highly responsive elements indicate that they are indeed critical for the kinase effect and are enriched in alternative exons. Interestingly, a CAAAAAA motif is sufficient for the PKA/CaMKIVregulated splicing of the exon 16 of the CaMK kinase 1 (CaMKK2) transcripts, implying a role of this motif in signaling cross-talk or feedback regulation between these kinases through alternative splicing. Therefore, these experiments identified a group of RNA elements responsive to PKA and CaMKIV from in vivo selection. This also provides an approach for selecting RNA elements similarly responsive to other cell signals controlling alternative splicing.Alternative pre-mRNA splicing is a common means of gene expression regulation in metazoans (1, 2). The expression of splice variants is controlled not only in a tissue-, sex-, or developmental stage-specific way but also in response to stimulation of particular protein kinases by extracellular factors (3-5).Many extracellular factors, including neurotransmitters and hormones, activate calcium and/or cAMP signals and downstream protein kinases. Besides membrane depolarization and the calcium/calmodulin-dependent protein kinase IV (CaMKIV) 4 (5-11), the cAMP-dependent protein kinase A (PKA) has also been implicated in the regulation of alternative splicing. For example, dopamine induces alternative splicing in striatal neurons through the dopamine receptor that activates the PKA pathway (12-14). PKA phosphorylates and colocalizes with arginine/serine-rich (SR) splicing factors and regulates alternative splicing of an E1A minigene (15). Alternative splicing in mammalian systems is controlled by multiple intronic and exonic regulatory elements called intronic or exonic splicing enhancers or silencers (1,16,17). The effect of a particular element can be either enhancer or silencer depending on its surrounding context in the pre-mRNA (5,(17)(18)(19). RNA elements that mediate cell signalregulated splicing have also been isolated. These elements include the A-rich element in the CD44 exon 5, the CaMKIVresponsive RNA elements (CaRRE1 and CaRRE2) in the STREX (stress axis-regulated exon) of Slo and exons 5 and 21 of the NMDAR1 (N-methyl-D-aspartic acid receptor, type I) ...