The significance of the flexible loop in the azurin (Az-iso2) from the obligate methylotroph Methylomonas sp. strain J.

The obligate methylotroph Methylomonas sp. strain J produces two azurins (Az-iso1 and Az-iso2) as candidates for electron acceptor from methylamine dehydrogenase (MADH) in the electron-transfer process involving the oxidation of methylamine to formaldehyde and ammonia. The X-ray crystallographic study indicated that Az-iso2 gives two types of crystals (form I and form II) with polyethylene glycol (PEG4000) and ammonium sulfate as the precipitants, respectively. Comparison between the two Az-iso2 structures in forms I and II reveals the remarkable structural changes at the top surface of the molecule around the copper atom. Az-iso2 possesses Gly43 instead of Val43 or Ala43, which is unique among all other azurins around the copper ligand His46, inducing the remarkable structural change in the loop region from Gly37 to Gly43. When the structure of Az-iso2 is superimposed on that of amicyanin in the ternary complex composed of MADH, amicyanin, and cytochrome c(551), the loop of Az-iso2 deeply overlaps with the light subunit of MADH. However, the Az-iso2 molecule is probably able to avoid any steric hindrance with the cognate MADH to form the complex for intermolecular electron-transfer reaction, since the loop containing Gly43 is flexible. We discuss why the electron-transfer activity of Az-iso2 is fivefold higher than that of Az-iso1.

AOX1c, a novel rice gene for alternative oxidase; comparison with rice AOX1a and AOX1b.

A novel gene for alternative oxidase (AOX) was isolated from rice (Oryza sativa L.) and characterized. The deduced amino acid sequence of the novel AOX gene contains features that are conserved among other AOXs. This AOX gene was designated AOX1c based on a phylogenetic analysis of the AOX genes. Northern hybridization analyses revealed that AOX1c and AOX1a/AOX1b transcripts accumulated differently in various rice organs and rice seedlings under low temperature conditions. AOX1c mRNA was mainly present in young leaves under constant light, mature leaves and panicles after heading, but it was not detected in young etiolated leaves and young roots of seedlings or young panicles. On the other hand, the mRNAs of the rice AOX1a and AOX1b genes were mainly present in young roots and mature leaves. Under low temperature conditions, the steady-state mRNA levels of the rice AOX1a and AOX1b genes clearly increased with time but the rice AOX1c gene was apparently not responsive to low temperature. The rice AOX gene family and differences in their regulation are discussed.