A chromatin code appears to mark introns and exons with distinct patterns of 
nucleosome enrichment and histone methylation. We investigated whether a causal 
relationship exists between splicing and chromatin modification by asking 
whether splice-site mutations affect the methylation of histone H3K36. Deletions 
of the 3' splice site in intron 2 or in both introns 1 and 2 of an integrated 
β-globin reporter gene caused a shift in relative distribution of H3K36 
trimethylation away from 5' ends and toward 3' ends. The effects of splice-site 
mutations correlated with enhanced retention of a U5 snRNP subunit on 
transcription complexes downstream of the gene. In contrast, a poly(A) site 
mutation did not affect H3K36 methylation. Similarly, global inhibition of 
splicing by spliceostatin A caused a rapid repositioning of H3K36me3 away from 
5' ends in favor of 3' ends. These results suggest that the cotranscriptional 
splicing apparatus influences establishment of normal patterns of histone 
modification.