Fig. 7

Working model for S2Lb activity with AtCOMPASS-like and SDG2 in histone H3-Lys4 trimethylation. In a major pathway, S2Lb and SDG2 target highly transcribed genes for H3-Lys4 trimethylation as part of a COMPASS-like complex acting independently of H2B monoubiquitination. S2Lb might as well be part of other COMPASS-like complexes with other histone methyltransferases such as ATX1 and ATXR7. Arabidopsis WDR5a, RbBP5-like (RBL), and Ash2-related (ASHR) are core components of AtCOMPASS-like complexes [37, 43,44,45] but the nature of their physical association with SDG2 remains to be determined. SDG2 might additionally act independently from S2Lb in different genic or cellular contexts, possibly with S2La. Whether histone H2B monoubiquitination is required for other types of At-COMPASS complexes activity and acts co-transcriptionally with PAF1c as in other species also remains to be resolved