Fig. 6

Model explaining the modulation of snoRNA expression. In this model, the abundance pattern of snoRNAs is determined by the host function and transcription pattern. Alternative initiation of transcription (either canonical YR-initiation or non-canonical YC-initiation) provides a means to independently regulate the expression of the snoRNA and its HG and is often found in UE and a few TE snoRNAs embedded in protein-coding HGs. On the other hand, simple transcription initiation is the hallmark of positively correlated snoRNA/HG pairs. Only the main pathways and features determining the snoRNA abundance patterns are illustrated. The anticorrelated, non-correlated, and positively correlated snoRNAs are indicated by −, 0, and +, respectively. Bold arrows represent the principal expression pathways adopted by snoRNAs. NMD, SI, and DI promoters stand for nonsense-mediated decay, simple-initiation, and dual-initiation promoters respectively