Histone H2A ubiquitination is a bulky posttranslational modification that occurs at the vicinity of the binding site for linker histones in the nucleosome. Therefore, we took several experimental approaches to investigate the role of ubiquitinated H2A (uH2A) in the binding of linker histones. Our results showed that uH2A was present in situ in histone H1-containing nucleosomes. Notably in vitro experiments using nucleosomes reconstituted onto 167-bp random sequence and 208-bp (5 S rRNA gene) DNA fragments showed that ubiquitination of H2A did not prevent binding of histone H1 but it rather enhanced the binding of this histone to the nucleosome. We also showed that ubiquitination of H2A did not affect the positioning of the histone octamer in the nucleosome in either the absence or the presence of linker histones.Despite the renewed interest in histone H2A/H2B ubiquitination (1-3), the functional role of uH2A 1 still remains controversial. This is in contrast to ubiquitinated H2B where a strong correlation with transcriptional activation has long been established (2), and even a molecular mechanism involving a transhistone regulatory pathway has been shown to be involved in this process (4, 5). With uH2A, there is almost as much experimental evidence for its association to actively transcribing chromatin as there is to the opposite. For instance, it was shown that nucleosomes from the transcriptionally poised hsp70 and copia genes from Drosophila contain 50% uH2A (6). Also nucleosomes containing mono-and diubiquitinated H2A were found to preferentially occur near the 5Ј-end of the transcribed mouse dihydrofolate reductase gene (7). On the other hand, early studies with rat liver nuclei linked the disappearance of uH2A to increased transcription (8), and it was shown that Drosophila nucleosomes consisting of non-transcribed 1.705 satellite DNA showed an enrichment in uH2A (9). In support of these early data, a role for histone uH2A in Polycomb silencing has been demonstrated recently (10). Furthermore uH2A has been shown to be concentrated in the heterochromatic inactive sex body in pachytene spermatocytes (11). Nevertheless the molecular mechanism(s) involved remains completely unknown.Despite the controversy, ubiquitinated histones have been associated with chromatin partially depleted of linker histones (12) and transcriptionally active or poised sequences (1, 2, 6, 7, 13) that are thought to be have a reduced linker histone content or have altered association with linker histones (for a review, see Ref. 14). However, there is evidence that suggests that histone H2A ubiquitination does not interfere with linker histone binding. Two independent studies have shown that H1 can be cross-linked to uH2A in vitro (15) and in vivo (16). Moreover, in the cross-linking studies performed in mouse cells, the molar ratio of H1-uH2A to H1-H2A was the same as the molar ratio of uH2A to H2A (16).Linker histones are required for the stabilization of well defined chromatin fibers (17-19), and interference with the binding of lin...