Abstract
Frugivorous and nectarivorous bats fuel their metabolism mostly by using carbohydrates and allocate the restricted amounts of ingested proteins mainly for anabolic protein syntheses rather than for catabolic energy production. Thus, it is possible that genes involved in protein (amino acid) catabolism may have undergone relaxed evolution in these fruit- and nectar-eating bats. The tyrosine aminotransferase (TAT, encoded by the Tat gene) is the rate-limiting enzyme in the tyrosine catabolic pathway. To test whether the Tat gene has undergone relaxed evolution in the fruit- and nectar-eating bats, we obtained the Tat coding region from 20 bat species including four Old World fruit bats (Pteropodidae) and two New World fruit bats (Phyllostomidae). Phylogenetic reconstructions revealed a gene tree in which all echolocating bats (including the New World fruit bats) formed a monophyletic group. The phylogenetic conflict appears to stem from accelerated TAT protein sequence evolution in the Old World fruit bats. Our molecular evolutionary analyses confirmed a change in the selection pressure acting on Tat, which was likely caused by a relaxation of the evolutionary constraints on the Tat gene in the Old World fruit bats. Hepatic TAT activity assays showed that TAT activities in species of the Old World fruit bats are significantly lower than those of insectivorous bats and omnivorous mice, which was not caused by a change in TAT protein levels in the liver. Our study provides unambiguous evidence that the Tat gene has undergone relaxed evolution in the Old World fruit bats in response to changes in their metabolism due to the evolution of their special diet.
Highlights
In mammals, the liver is the major organ involved with amino acid metabolism
Relaxation of the evolutionary constraints on the genes encoding PAH, hydroxyphenylpyruvic acid dioxygenase (HPD) and HGD were detected in Old World fruit bats, including some amino acid substitutions that should cause adverse effects to their enzymatic activity [37]. All these results strongly indicated that the Old World fruit bats have reduced their reliance on the phenylalanine and tyrosine catabolic pathway in relation to their special diet and associated metabolism, leading us to speculate that the Tat gene, which is involved in the ratelimiting step of tyrosine catabolism, may have undergone relaxed evolution in fruit- and nectar-eating bats
We sequenced the Tat coding region and compared the evolution of the sequences between frugivorous and insectivorous bats
Summary
The liver is the major organ involved with amino acid metabolism. In addition to protein synthesis, tyrosine (Tyr) is catabolized to acetoacetate and fumarate for energy production by the tyrosine catabolic pathway, which includes four steps that are catalyzed in turn by tyrosine aminotransferase (TAT), 4-hydroxyphenylpyruvic acid dioxygenase (HPD), homogentisate 1,2dioxygenase (HGD) and fumarylacetoacetase (FAH) [1]. TAT (encoded by the Tat gene) is the rate-limiting enzyme in this pathway [2] and catalyzes the conversion of tyrosine to phydroxyphenylpyruvate. Since the accumulation of tyrosine in blood causes toxic effects to tissues and organs [7], the breakdown of tyrosine by TAT is very important for human health. Deficiency of TAT, which is caused by genetic mutations in the Tat gene, in humans leads to tyrosinemia type II syndrome characterized by elevated blood tyrosine levels, mental retardation, etc [8]. More than 15 distinct mutations [8,9,10,11,12,13,14,15] have been identified in humans that cause tyrosinemia type II
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