Unlike most eubacteria, Streptomyces species usually contain linear chromosomes and plasmids (9,16,17). The linear plasmids are 12 to 1,700 kb long (16,27). Their telomeres contain inverted repeat sequences from 44 bp (7) to 180 kb (19), and their 5Ј telomeric ends are linked covalently to terminal proteins (1, 28). Unlike linear replicons of adenoviruses and bacteriophage 29, which also contain terminal proteins linking covalently to 5Ј telomeric DNA ends and undergo replication by a mechanism of strand displacement (23), replication of Streptomyces linear plasmids starts at centrally located loci (5, 25) and continues bidirectionally towards the telomeres-leaving an ϳ280-nucleotide 3Ј single-strand overhang as an intermediate (5). This is converted to a double strand by a postulated "folding back" of multiply short palindromes on the telomere extension (20). The chromosomal telomere-associated protein (TapL, encoded by tapL) binds to the palindromes II/III then to recruit telomere terminal protein (TpgL, encoded by tpgL) (1, 2). Neither tapL nor tpgL homologous genes are carried by the linear plasmids pSLA2 and pSCL1 (1, 2), and the mechanism of recruitment or activation of these chromosomal telomere proteins for plasmid telomere replication is unknown.The centrally located loci of Streptomyces linear plasmids can also maintain propagation in circular mode when the telomeres are deleted (5,8,22,25). The centrally located locus for replication of linear plasmid pSLA2 is composed of the iterons located within the essential genes rep1 (encoding DNA-binding protein) and rep2 (DNA helicase) (6). Experimental evidence shows that the replication origin of linear SCP1 plasmids contains a rep2 pSLA2 -like gene and its adjacent regions contain different iteron sequences (22). Similar loci are also indicated in linear plasmids pSCL1 and SLP2 (11,27). The extent of functional similarity of these individual replicating components, and consequently the extent to which mechanisms of replication are similar among linear plasmids, is not known.The minimal locus required for maintaining the replication of pSLA2 in circular mode cannot allow its propagation in linear mode unless it also contains a new plasmid locus, rlrA pSLA2 (required for linear replication) (21). Plasmids containing rlrA pSLA2 are detrimental for propagation in circular mode, the effect of which can be reversed by an adjacent and divergently transcribed locus, rorA pSLA2 (rlrA override), which resembles korA (kilA override) of Streptomyces circular plasmid pIJ101 (14,26). rlrA pSLA2 and rorA pSLA2 increase inheritance and copy number of pSLA2 circular plasmids, suggesting that they may affect the origin locus (e.g., iterons) (21). Although rorA pSLA2 -homologous genes are found in linear plasmids SCP1, pSLA2-L, and SLP2, no rlrA pSLA2 -homologous loci of the plasmids are found (3,11,18), suggesting that they carry a distinct locus that enables replication in linear mode.SLP2 is a large (50,410-bp) linear plasmid of Streptomyces lividans (7, 10, 11). Here we r...
Apramycin is unique in the aminoglycoside family due to its octodiose moiety. However, either the biosynthesis process or the precursors involved are largely unknown. Addition of glycine, as well as serine or threonine, to the Streptomyces tenebrabrius UD2 fermentation medium substantially increases the production of apramycin with little effect on the growth of mycelia, indicating that glycine and/or serine might be involved in the biosynthesis of apramycin. The 13C-NMR analysis of [2-13C] glycine-fed (25% enrichment) apramycin showed that glycine specifically and efficiently incorporated into the only N-CH3 substituent of apramycin on the C7' of the octodiose moiety. We noticed that the in vivo concentration of S-adenosyl methionine increased in parallel with the addition of glycine, while the addition of methione in the fermentation medium significantly decreased the productivity of apramycin. Therefore, the methyl donor function of glycine is proposed to be involved in the methionine cycle but methionine itself was proposed to inhibit the methylation and methyl transfer processes a previously reported for the case of rapamycin. The 15N NMR spectra of [2-13C,15N]serine labeled apramycin indicated that serine may also act as a limiting precursor contributing to the -NH2 substituents of apramycin.
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