A-MYB substitutes for B-MYB in activating cell cycle genes and in stimulating proliferation

A-MYB (MYBL1) is a transcription factor with a role in meiosis in spermatocytes. The related B-MYB protein is a key proto-oncogene and a master regulator activating late cell cycle genes. To activate genes, B-MYB forms a complex with MuvB and is recruited indirectly to cell cycle genes homology region (CHR) promoter sites of target genes. Activation through the B-MYB-MuvB (MMB) complex is essential for successful mitosis. Here, we discover that A-MYB has a function in transcriptional regulation of the mitotic cell cycle and can substitute for B-MYB. Knockdown experiments in cells not related to spermatogenesis show that B-MYB loss alone only delays cell cycle progression. Only dual knockdown of B-MYB and A-MYB causes cell cycle arrest. A-MYB can substitute for B-MYB in binding to MuvB. The resulting A-MYB-MuvB complex activates genes through CHR sites. We find that A-MYB activates the same target genes as B-MYB. Many of the corresponding proteins are central regulators of the cell division cycle. In summary, we demonstrate that A-MYB is an activator of the mitotic cell cycle by activating late cell cycle genes. Overall design: To investigate the function of A-MYB and B-MYB in the mitotic cell cycle, siRNA knockdowns were performed on HCT116 cells as well as U2OS cells. Cells were treated with siRNA targeting MYBL1 (A-MYB), or MYBL2 (B-MYB) as well as a combination of MYBL1 and MYBL2 targeting siRNAs for 48 h. Non-targeting siRNA was used as a negative control. Total RNA expression was analzed by paired-end RNA-seq. Per cell line and sample two biological replicates were generated.