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Exoribonucleases are enzymes that cleave RNA molecules by removing terminal nucleotides from the 3′ or 5′ end of the RNA molecules. They are key factors in RNA metabolism and have a relevant role in the processing and degradation of all types of RNAs. The 3′ to 5′ exoribonucleases are divided into families, according to their sequence and structural characteristics. The PDX family contains phosphate-dependent degradative enzymes, which can also perform the synthesis of RNA tails when phosphate is limiting. The RNB family contains hydrolytic enzymes with a similar domain organization. All proteins from this widespread family present the characteristic RNB domain responsible for the 3′ to 5′ exoribonuclease activity. In eukaryotes they can act alone or in a complex, the exosome, where they are the only active component. Finally, the DEDD family includes both RNA and DNA exonucleases and they present a similar mechanism of action. In this chapter, we will summarize the available information regarding the 3′ to 5′ exoribonucleases and discuss their importance for the RNA metabolism.

The 5′-3′ exoribonucleases have important functions in RNA processing, RNA degradation, RNA interference, transcription, and other cellular processes. The Xrn1 and Xrn2/Rat1 family of enzymes are the best characterized 5′-3′ exoribonucleases, and there has been significant recent progress in the understanding of their structure and function. Especially, the first structural information on Rat1 just became available. Other 5′-3′ exoribonucleases have been identified recently, including yeast Rrp17 and B. subtilis RNase J1, the first enzyme with 5′-3′ exoribonuclease activity found in prokaryotes. This review will summarize our current understanding of these enzymes, focusing on their sequence conservation, molecular structure, biochemical and cellular functions.

The means by which Leishmania survive within their several hosts remain unknown. However, as their surface membrane interfaces with host environments, it must fulfill numerous vital functions requisite for maintaining the parasite. Thus, knowledge of the biochemistry and physiology of these membranes is relevant to defining the mechanisms of parasite survival. This topic has been reviewed recently (Dwyer & Gottlieb, 1983, 1985) and the current report concerns some recent advances in this area.