ALTERNATIVE SPLICING OF THE CHICKEN C-MYB EXON-9A

The c-myb gene products are thought to be regulators of cellular repli cation and of differentiation and heterogeneity may underlie their mul tiple functions. To investigate the possible existence of heterogeneit y we have examined the chicken c-myb mRNAs by Northern blot analysis a nd polymerase chain reaction amplification of cDNAs (RT-PCR). Northern blot analysis with the c-myb cDNA clone pSG3, which contains the enti re open reading frame (ORF) plus 500 base pairs of 3' untranslated seq uences (Gerondakis & Bishop, 1986), and genomic probes revealed c-myb RNA species of 4.3 kb in addition to the major 4.0 kb species. The 4.3 kb c-myb RNA contained the alternatively spliced exon 9A which is hig hly conserved and has also been detected in a minor 4.3 kb alternative ly spliced c-myb mRNA in murine and human cells. Sequencing of the avi an exon 9A revealed 360 bp exon homologous to that found in murine and human mRNAs, which contains three highly conserved sequence regions s hared by all three species. RT-PCR demonstrated usage of exon 9A in fi ve hematopoietic tissues and revealed an additional splicing variant w hich used the 3' portion of exon 9A. Northern blot analysis using spli ce site-specific oligonucleotide probes spanning the two splice juncti ons between exon 9 and 9A revealed four additional c-myb RNAs of 4.4 k b, 2.2 kb, 2.0 kb and 1.4 kb.