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