Attached growth reactor
A reactor in which the microorganisms are engineered surfaces within the reactor. Examples growth reactors are the trickling filter and the rotating biological contactor. See suspended growth reactor.

ATG or AUG
The codon for methionine; the translation initiation codon.  Usually, protein translation can only start at a methionine coden (although this codon may be found elsewhere within the  protein sequence as well). In eukaryotic DNAis ATG; in RNA it is AUG. Usually, the first mRNA is the point at which translation starts,  and an open reading frame  follows - i.e. the nucleotides taken three at a time will code for the amino acids of the protein, and a stop  codon will be found only when the protein coding region is  complete.

ATP-ase
An enzyme that brings about the hydrolysis of ATP, by the cleavage of either one phosphate group with the ADP and inorganic phosphate, or of two phosphate groups with the formation of AMP and pyrophosphate.

Attenuated vaccine
A virulent organism that has been modified to produce a less  virulent form, but nevertheless retains ability to elicit antibodies the against the virulent form.
Attenuation
A mechanism for controlling gene expression in prokaryote that involves premature termination of transcription.

Attenuator
A nucleotide sequence in the 5' region of a prokaryotic gene (or in its RNA) that causes premature termination of transcription, possibly by forming a secondary structure.