Breeding to increase beta-carotene levels in cereal grains, termed provitamin A biofortification, is an economical approach to address dietary vitamin A deficiency in the developing world. Experimental evidence from association and linkage populations in maize (Zea mays L.) demonstrate that the gene encoding beta-carotene hydroxylase 1 (crtRB1) underlies a principal quantitative trait locus associated with beta-carotene concentration and conversion in maize kernels. crtRB1 alleles associated with reduced transcript expression correlate with higher beta-carotene concentrations. Genetic variation at crtRB1 also affects hydroxylation efficiency among encoded allozymes, as observed by resultant carotenoid profiles in recombinant expression assays. The most favorable crtRB1 alleles, rare in frequency and unique to temperate germplasm, are being introgressed via inexpensive PCR marker-assisted selection into tropical maize germplasm adapted to developing countries, where it is most needed for human health.
APOBECs are a family of cytidine deaminases involved in various important biological processes such as antibody diversification/maturation, restriction of viral infection, and generation of somatic mutations. Catalytically active APOBEC proteins execute their biological functions mostly through deaminating cytosine (C) to uracil on ssDNA/RNA. Activation-induced cytidine deaminase (AID), one of the APOBEC members, was reported to deaminate methylated cytosine (mC) on DNA and this mC deamination was proposed to be involved in demethylation of mC for epigenetic regulation. The mC deamination activity is later demonstrated for APOBEC3A (A3A), and more recently for APOBEC3B (A3B) and APOBEC3H (A3H). Despite extensive studies on APOBEC proteins, questions regarding whether the rest of APOBEC members have any mC deaminase activity and what are the relative deaminase activities for each APOBEC member remain unclear. Here we performed a family-wide analysis of deaminase activities on C and mC by using purified recombinant proteins for eleven known human APOBEC proteins under similar conditions. Our comprehensive analyses revealed each APOBEC has unique deaminase activity and selectivity for mC. A3A and A3H showed distinctively high deaminase activities on C and mC with relatively high selectivity for mC, whereas six other APOBEC members showed relatively low deaminase activity and selectivity for mC. Our mutational analysis showed that loop-1 of A3A is responsible for its high deaminase activity and selectivity for mC. These findings extend our understanding of APOBEC family proteins that have important roles in diverse biological functions as well as in genetic mutations.
It is unclear whether both CD1 and CD2 of A3B can deaminate C and mC. We showed that only CD2 is active, and has weak mC deaminating activity. We successfully engineered A3BCD2 to gain over 100-fold higher mC deamination activity.
F plasmid oriT DNA extending from the F kilobase coordinate 66.7 (base pair [bp] 1 on the oriT sequence map) rightward to bp 527 was analyzed for intrinsic bends (by permutation assays) and for binding of integration host factor (IHF) (by gel retardation and DNase footprinting). Intrinsic bending of the 527-bp fragment (bend center approximately at bp 240) was represented as a composite of at least two components located near bp 170 and near bp 260. IHF bound primarily to a site extending from bp 165 to 195 and with lower affinity to a site extending from bp 287 to 319. The intrinsic curvature and sequences to which IHF binds (IHF is known to bend DNA) may play a structural role in oriT function.
Functional domains of the Escherichia coli F plasmid oniT locus were identified by deletion analysis. DNA sequences required for nicking or transfer were revealed by cloning deleted segments of oriT into otherwise nonmobilizable pUC8 vectors and testing for their ability to promote transfer or to be nicked when tra operon functions were provided in trans. Removal of DNA sequences to the right of the central A+T-rich region (i.e., from the direction of traM) did not affect the susceptibility of oniT to nicking functions; however, transfer efficiency for oriT segments deleted from the right was progressively reduced over an 80-to 100-bp interval. Deletions extending toward the oniT nick site from the left did not affect the frequency of transfer if deletion endpoints lay at least 22 bp away from the nick site. Deletions or insertions in the central, A+T-rich region caused periodic variation in transfer efficiency, indicating that phase relationships between nicking and transfer domains of oriT must be preserved for full oriT function. These data show that the F oriT locus is extensive, with domains that individually contribute to transfer, nicking, and overall structure.
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