Excess of microRNAs in large and very 5′ biased introns

Many of microRNAs (miRNAs) and small nucleolar RNAs (snoRNAs) are located within the introns of genes in eukaryotes. Contrary to intronic snoRNAs, intronic miRNAs are processed from unspliced intronic regions before the catalysis of splicing in vertebrates. By analyzing the distribution patterns of the length and position of the introns hosting these two groups of small RNA genes, we observed that both human and mouse intronic miRNAs tended to be present in large introns, and miRNA host introns have a more 5′-biased position distribution compared with all other introns among the two genomes. These observations indicate that the negative selection of functional constraints might affect the intron size in both genomes. Interestingly, the very 5′-biased positions of miRNA host introns may be necessary for the transcription and regulation of intronic miRNAs to utilize the regulatory signals within the 5′-UTRs of their host genes.