A tidally distorted dwarf galaxy near NGC 4449

NGC 4449 is a nearby Magellanic irregular starburst galaxy with a B-band absolute magnitude of -18 and a prominent, massive, intermediate-age nucleus at a distance from Earth of 3.8?megaparsecs (ref. 3). It is wreathed in an extraordinary neutral hydrogen (H?I) complex, which includes rings, shells and a counter-rotating core, spanning ～90?kiloparsecs (kpc; refs 1, 4). NGC 4449 is relatively isolated, although an interaction with its nearest known companion--the galaxy DDO 125, some 40?kpc to the south--has been proposed as being responsible for the complexity of its H?I structure. Here we report the presence of a dwarf galaxy companion to NGC 4449, namely NGC 4449B. This companion has a V-band absolute magnitude of -13.4 and a half-light radius of 2.7?kpc, with a full extent of around 8?kpc. It is in a transient stage of tidal disruption, similar to that of the Sagittarius dwarf near the Milky Way. NGC 4449B exhibits a striking S-shaped morphology that has been predicted for disrupting galaxies but has hitherto been seen only in a dissolving globular cluster. We also detect an additional arc or disk ripple embedded in a two-component stellar halo, including a component extending twice as far as previously known, to about 20?kpc from the galaxy's centre.     

Direct charging of tRNA(CUA) with pyrrolysine in vitro and in vivo

Pyrrolysine is the 22nd amino acid. An unresolved question has been how this atypical genetically encoded residue is inserted into proteins, because all previously described naturally occurring aminoacyl-tRNA synthetases are specific for one of the 20 universally distributed amino acids. Here we establish that synthetic L-pyrrolysine is attached as a free molecule to tRNA(CUA) by PylS, an archaeal class II aminoacyl-tRNA synthetase. PylS activates pyrrolysine with ATP and ligates pyrrolysine to tRNA(CUA) in vitro in reactions specific for pyrrolysine. The addition of pyrrolysine to Escherichia coli cells expressing pylT (encoding tRNA(CUA)) and pylS results in the translation of UAG in vivo as a sense codon. This is the first example from nature of direct aminoacylation of a tRNA with a non-canonical amino acid and shows that the genetic code of E. coli can be expanded to include UAG-directed pyrrolysine incorporation into proteins.