Recent years witnessed the discovery of ubiquitous and diverse 5′-end RNA cap-like modifications in prokaryotes as well as in eukaryotes. These non-canonical caps include metabolic cofactors, such as NAD⁺/NADH, FAD, cell wall precursors UDP-GlcNAc, alarmones, e.g. dinucleotides polyphosphates, ADP-ribose and potentially other nucleoside derivatives. They are installed at the 5′ position of RNA via template-dependent incorporation of nucleotide analogues as an initiation substrate by RNA polymerases. However, the discovery of NAD-capped processed RNAs in human cells suggests the existence of alternative post-transcriptional NC capping pathways. In this review, we compiled growing evidence for a number of these other mechanisms which produce various non-canonically capped RNAs and a growing repertoire of capping small molecules. Enzymes shown to be involved are ADP-ribose polymerases, glycohydrolases and tRNA synthetases, and may potentially include RNA 3′-phosphate cyclases, tRNA guanylyl transferases, RNA ligases and ribozymes. An emerging rich variety of capping molecules and enzymes suggests an unrecognized level of complexity of RNA metabolism.

近年来，在原核生物和真核生物中发现了普遍存​​在且多样的5'-端RNA帽样修饰。这些非典型上限包括代谢辅因子，例如NAD ⁺/ NADH，FAD，细胞壁前体UDP-GlcNAc，警报蛋白，例如二磷酸多核苷酸，ADP-核糖和可能的其他核苷衍生物。通过RNA聚合酶将核苷酸类似物作为起始底物进行模板依赖性掺入，将它们安装在RNA的5'位置。但是，在人类细胞中发现了NAD封端的加工RNA，这表明存在替代的转录后NC覆盖途径。在这篇综述中，我们为许多其他机制产生了越来越多的证据，这些机制产生了各种非经典的加帽RNA，以及越来越多的加帽小分子。显示涉及的酶是ADP-核糖聚合酶，糖水解酶和tRNA合成酶，并且可能包括RNA 3'-磷酸环化酶，tRNA鸟苷基转移酶，RNA连接酶和核酶。