Abstract
The frequency of thermal ‘dark events’ in the membrane current of rhodopsin rods of the bullfrog,Rana catesbeiana, is considerably lower than observed in rods of two toad species, even though all three rhodopsins have approximately the same absorbance characteristics. In order to map amino acid substitutions possibly associated with thermal stability in the genusRana,the cDNA's coding for the rhodopsins ofBufo bufo, B. marinusandR. temporariawere sequenced and the conceptually translated protein sequences aligned to the previously sequenced rhodopsins ofR. catesbeiana, R. pipiensandXenopus laevis.Across the six anuran species studied, there are sixteen non-conserved substitutions and six changes that include gain or loss of a hydroxyl group. Serine or threonine at position 220 is unique to the threeRanaspecies, phenylalanine at position 270 is unique to all threeRanasand toX. laevis,and phenylalanine at position 274 is unique to both species of the genusBufo.This investigation produces a list of substitutions that are candidates for future studies of thermal stability. In addition, a number of amino acids are identified that apparently donotinfluence absorbance characteristics, at least not cumulatively.
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