RNA helicase that acts as a sensor of the transcriptional status of both RNA polymerase (Pol) I and II: promotes ribosomal RNA (rRNA) processing and transcription from polymerase II (Pol II) (By similarity). Binds various RNAs, such as rRNAs, snoRNAs, 7SK and, at lower extent, mRNAs (By similarity). In the nucleolus, localizes to rDNA locus, where it directly binds rRNAs and snoRNAs, and promotes rRNA transcription, processing and modification (PubMed:12851405) (Probable). Required for rRNA 2'-O-methylation, possibly by promoting the recruitment of late-acting snoRNAs SNORD56 and SNORD58 with pre-ribosomal complexes (By similarity). In the nucleoplasm, binds 7SK RNA and is recruited to the promoters of Pol II-transcribed genes: acts by facilitating the release of P-TEFb from inhibitory 7SK snRNP in a manner that is dependent on its helicase activity, thereby promoting transcription of its target genes (By similarity). Required to prevent R-loop-associated DNA damage and transcription-associated genomic instability (By similarity). ATP + H2O = ADP + H(+) + phosphate Widely expressed. Expressed at higher level in stomach. Expressed at higher level compared to ddx21-b. Belongs to the DEAD box helicase family. DDX21/DDX50 subfamily. Truncated N-terminus.