This paper advances the conceptualization of STEM teacher professional knowledge reflecting the specialized knowledge needed to support English learner (EL) language development during science and mathematics instruction. What distinguishes this model is the integration of conceptual understanding, a key feature of subject matter knowledge, with disciplinary literacy knowledge (DLK). Several models have been proposed to capture the knowledge, skills, and dispositions of effective teachers. However, few studies have empirically investigated the model's utility in producing effective EL instruction. Two professional development (PD) approaches were tested. One approach explicitly focused on developing subject matter knowledge and the second approach focused on developing pedagogical knowledge without an explicit attention to subject matter knowledge. Both approaches included PD on DLK. Outcome variables included teacher math and science content knowledge and instructional quality. Findings show overall science content knowledge score increased for both PD approaches, but no significant differences in overall math content knowledge score were found. This pattern suggests that for elementary teachers, prolonged PD can improve teacher science knowledge. Significant increases in instructional quality were found only for teachers who received PD with an explicit focus subject matter. PD focused on pedagogical knowledge did not improve overall teaching quality. Implications are discussed.