Figure 1

Ordered assembly of MSL proteins and roX RNA increases stability of the DCC. The first step of assembly involves the recognition of about 35 chromosomal entry sites by a preDCC consisting of MSL1 and MSL2. This recognition does not require any of the known roX RNAs. In the next step, roX2 and MLE enter the complex to form a more stable primary DCC (PrimDCC) at these 35 sites. Spreading throughout the entire X chromosome requires the formation of the complete complex by addition of MOF and MSL3. The sequential addition of new components, particularly the roX2 RNA, might induce changes in the structure of already incorporated components (illustrated by ovals becoming circles), increasing the stability of the DCC. MLE might be removed in vitro (for example, by elevated ionic strength) without destroying the entire complex, because of the stabilizing presence of MOF-MSL3. The role of roX1 RNA is not clear, but it is integrated late in this process, together with or after MOF and MSL3; roX1 might provide additional stability to the mature DCC (MatDCC). So, roX1 and roX2 might be partially redundant.