Peculiarities of Non Coding Micrornas Organization in H. Sapiens and some other Species

In 1993 Victor Ambros and colleagues discovered that lin4, a gene known to control the of C.elegans larval development produces a pair of small RNAs [1,2]. For seven years after this discovery, there was no evidence for lin-4-like RNAs beyond nematodes. This all changed upon the discovery that let-7, another gene in the C.elegans encoded a second ~22 nt regulatory RNA. Homolog’s of the let-7 gene were soon identified in human and fly genomes, and let-7 RNA itself was detected in human, Drosophila, and eleven other bilateral animals [3]. One year later, a total of over one hundred additional genes for tiny non coding RNAs were detected. The RNA products of these genes resembled the lin-4 and let-7 RNAs. They were ~22nt endogenously expressed RNAs and potentially processed from one arm of a stem loop precursor. They were generally conserved in evolution-some quite broadly, others only in more closely related species. But unlike lin-4 and let-7 RNAs, many of them were not expressed in distinct stages of development but were more likely to be expressed in particular cell types. Intensified cloning efforts have revealed numerous additional mi RNA genes in mammals, fish, worms, and flies [3-7]. The shorter lin-4 RNA is now recognized as the founding member of an abundant class of tiny regulatory RNAs called micro RNAs or mi RNAs [3,4]. Mi RNA functions include control of cell proliferation, cell death, and fat metabolism in flies [8,9], neuronal patterning in nematodes [10], modulation of hematopoietic lineage differentiation in mammals [11,12], and control of leaf and flower development in plants. Since the report of the lin-4 RNA and its regulation of lin-14 the major topics discussed were mi RNA genomics, mi RNA biogenesis, and mi RNA regulatory mechanisms [13].