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