N-Phosphorylated guanidino compounds, commonly referred to as phosphagens, play a critical role as an energy reserve because of the high energy phosphate that can be transferred when the renewal of ATP is needed. It has also been proposed that these compounds function in spatial buffering of cellular energy production sites. So, phosphagens act as reserves not only of ATP but also of inorganic phosphate, which is mostly returned to the medium by metabolic consumption of ATP. Phosphoarginine is the main reserve of high energy phosphate compounds in a wide variety of invertebrates. In addition phosphocreatine, phosphoglycocyamine, phosphotaurocyamine, phosphohypotaurocyamine, phosphoopheline, and phospholombricine are also found, whereas in vertebrates the only one present is phosphocreatine (1, 2).Arginine kinase (EC 2.7.3.3) is a member of a conserved family of phosphotransferases which also includes creatine kinase. These enzymes catalyze the reversible transfer of a phosphoryl group from ATP to a guanidino acceptor, which can be either an amino acid (e.g. lombricine or arginine) or a carboxylate (e.g. creatine or glycocyamine; Reaction 1).

RNA duplexes containing the modified base 2-aminoadenine in place of adenine are stabilized through the formation of three hydrogen bonds in 2-amino A* U base pairs. Antisense 2'-0-alkyloligoribonucleotide probes incorporating 2-aminoadenosine are thus able to efficiently affinity select RNP particles which are otherwise inaccessible. This has allowed the efficient and specific depletion of U5 snRNP from HeLa cell nuclear splicing extracts. U5 snRNP is shown to be essential for spliceosome assembly and for both steps of pre-mRNA splicing. The absence of U5 snRNP prevents the stable association of U4/U6 but not Ul and U2 snRNPs with pre-mRNA. INTRODUCTIONThe splicing of nuclear messenger RNA precursors (pre-mRNA) occurs within a multicomponent structure termed the spliceosome. The major components of the spliceosome belong to the U-class of small nuclear ribonucleoprotein particles (U snRNPs), specifically Ul, U2, U4/U6 and U5 snRNPs. These four snRNPs, together with non-snRNP protein factors, assemble along an ordered pathway to form a functional spliceosome (for recent reviews see 1-4). A combination of affinity selection methods and native gel analyses have shown that the U1, U2, U4/U6 and U5 snRNPs are present in splicing complexes (5)(6)(7)(8)(9)(10)(11)(12)(13)(14). In order to study the roles of the different snRNP particles in the splicing mechanism, a variety of methods have been used to inactivate individual snRNP particles in in vitro splicing extracts. These include inhibition by snRNP-specific antibodies (15-17), site-specific cleavage of snRNA components by RNase H in the presence of complementary DNA oligonucleotides (17-21), and masking specific snRNA domains using antisense 2'-0-methyl oligoribonucleotides (2'-OMe RNA) (22 -24). In Saccharomyces cerevisiae it has also been possible to exploit a genetic approach in which expression of a specific snRNA gene is placed under the control of the inducible p3-gal promoter. This has allowed in vivo depletion of specific snRNP species (25 -27). Using one or more of the above methods, U 1,

In this review the current state-of-the-art of S-adenosylmethionine (SAM)-dependent methyltransferases and SAM are evaluated. Their structural classification and diversity is introduced and key mechanistic aspects presented which are then detailed further. Then, catalytic SAM as a target for drugs, and approaches to utilise SAM as a cofactor in synthesis are introduced with different supply and regeneration approaches evaluated. The use of SAM analogues are also described. Finally O-, N-, C-and S-MTs, their synthetic applications and potential for compound diversification is given.