These reports have suggested that {TAIL ~ the SIR2 proteins} are {HEAD ~ either histone} ADP ribosyltransferases , or histone/protein deacetylases , or both  
These reports have suggested that { TAIL ~ the SIR2 proteins } are { HEAD ~ either histone } ADP ribosyltransferases , or histone / protein deacetylases , or both
OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH