Horizontal transfer of genes between species is an important mechanism for bacterial genome evolution. In Escherichia coli, conjugation is the transfer from a donor (F(+)) to a recipient (F(-)) cell through cell-to-cell contact. We demonstrate what we believe to be a novel qPCR method for quantifying the transfer kinetics of the F plasmid in a population by enumerating the relative abundance of genetic loci unique to the plasmid and the chromosome. This approach allows us to query the plasmid transfer rate without the need for selective culturing with unprecedented single locus resolution. We fit the results to a mass action model where the rate of plasmid growth includes the lag time of newly formed F(+) transconjugants and the recovery time between successive conjugation events of the F(+) donors. By assaying defined mixtures of genotypically identical donor and recipient cells at constant inoculation densities, we extract an F plasmid transfer rate of 5 × 10(-10) (cells/mL · min)(-1). We confirm a plasmid/chromosome ratio of 1:1 in homogenous F(+) populations throughout batch growth. Surprisingly, in some mixture experiments we observe an excess of F plasmid in the early saturation phase that equilibrates to a final ratio of one plasmid per chromosome.

No abstract

No abstract

Specification of the dorsoventral (DV) axis is critical for the subsequent differentiation of regional fate in the primary germ layers of the vertebrate embryo. We have identified a novel factor that is essential for dorsal development in embryos of the frog Xenopus laevis. Misexpression of Xenopus mab21-like 3 (Xmab21l3) dorsalizes gastrula-stage mesoderm and neurula-stage ectoderm, while morpholino-mediated knockdown of Xmab21l3 inhibits dorsal differentiation of these embryonic germ layers. Xmab21l3 is a member of a chordate-specific subclass of a recently characterized gene family, all members of which contain a conserved, but as yet ill-defined, Mab21 domain. Our studies suggest that Xmab21l3 functions to repress ventralizing activity in the early vertebrate embryo, via BMP/Smad and Ras/ERK signaling.