Disclosed herein are methods and compositions of matter that enable the biological conversion of gaseous waste streams (CO2, stranded natural gas, flue gas, biogas, landfill gas, etc.) to a platform chemical, 3-hydroxybutyrate, which can in turn be upgraded to fuels and polymers (e.g. polypropylene and polymers). The technology thus presents both a mechanism to valorize gaseous waste streams and establish sustainable production routes to chemicals and plastics via the overexpression of PHB depolymerase while knocking out the AACS pathway in this specific strain of methanotrophic bacteria.