Tri-methylated H3 lysine 4 (H3K4me3) is associated with transcriptionally active 
genes, but its function in the transcription process is still unclear. Point 
mutations in the catalytic domain of ATX1 (ARABIDOPSIS TRITHORAX1), a H3K4 
methyltransferase, and RNAi knockdowns of subunits of the AtCOMPASS-like 
(Arabidopsis Complex Proteins Associated with Set) were used to address this 
question. We demonstrate that both ATX1 and AtCOMPASS-like are required for high 
level accumulation of TBP (TATA-binding protein) and Pol II at promoters and 
that this requirement is independent of the catalytic histone modifying 
activity. However, the catalytic function is critically required for 
transcription as H3K4me3 levels determine the efficiency of transcription 
elongation. The roles of H3K4me3, ATX1, and AtCOMPASS-like may be of a general 
relevance for transcription of Trithorax-activated eukaryotic genes.